Transversely-excited film bulk acoustic resonator with optimized piezoelectric plate thickness

ABSTRACT

Acoustic resonators and filter devices. An acoustic resonator includes a piezoelectric plate having front and back surfaces, a portion of the piezoelectric plate forming a diaphragm, and a conductor pattern on the front surface, the conductor pattern including a multi-mark interdigital transducer (IDT), fingers of the IDT on the diaphragm. A thickness between the front and back surfaces is greater than or equal to 200 nm and less than or equal to 1000 nm.

RELATED APPLICATION INFORMATION

This patent is a continuation of application Ser. No. 17/560,889, filedDec. 23, 2021, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATOR WITH OPTIMIZED ELECTRODE THICKNESS, MARK, AND PITCH.

Application Ser. No. 17/560,889 is a continuation-in-part of applicationSer. No. 17/022,042, filed Sep. 15, 2020, entitled TRANSVERSELY-EXCITEDFILM BULK ACOUSTIC RESONATOR USING YX-CUT LITHIUM NIOBATE FOR HIGH POWERAPPLICATIONS, which claims priority from provisional patent application63/026,824, filed May 19, 2020, entitled IDT SIDEWALL ANGLE TO CONTROLSPURIOUS MODES IN XBARS.

Application Ser. No. 17/022,042 is also continuation-in-part ofapplication Ser. No. 16/829,617, entitled HIGH POWERTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS ON Z-CUT LITHIUMNIOBATE, filed Mar. 25, 2020, now U.S. Pat. No. 10,868,512, which is acontinuation of application Ser. No. 16/578,811, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS FOR HIGH POWERAPPLICATIONS, filed Sep. 23, 2019, now U.S. Pat. No. 10,637,438.

Application Ser. No. 17/022,042 is also a continuation in part ofapplication Ser. No. 16/782,971, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR USING ROTATED Y-X CUT LITHIUM NIOBATE, filed Feb. 5,2020, now U.S. Pat. No. 10,790,802, which claims priority from62/904,133, filed Sep. 23, 2019, entitled WIDE BAND BAW RESONATORS ON120-130 Y-X LITHIUM NIOBATE SUBSTRATES; is a continuation-in-part ofapplication Ser. No. 16/689,707, entitled BANDPASS FILTER WITH FREQUENCYSEPARATION BETWEEN SHUNT AND SERIES RESONATORS SET BY DIELECTRIC LAYERTHICKNESS, filed Nov. 20, 2019, now U.S. Pat. No. 10,917,070; and is acontinuation in part of application Ser. No. 16,438,141, filed Jun. 11,2019, entitled SOLIDLY-MOUNTED TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATOR, now U.S. Pat. No. 10,601,392, which claims priority fromapplication 62/818,564, filed Mar. 14, 2019, entitled SOLIDLY MOUNTEDXBAR and application 62/753,809, filed Oct. 31, 2018, entitled SOLIDLYMOUNTED SHEAR-MODE FILM BULK ACOUSTIC RESONATOR. Application Ser. No.16/438,141 is also a continuation-in-part of application Ser. No.16/230,443. Application Ser. No. 16/578,811 is a continuation-in-part ofapplication Ser. No. 16/230,443 and application Ser. No. 16/689,707 is acontinuation of application Ser. No. 16/230,443. Application Ser. No.16/230,443, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR,filed Dec. 21, 2018, now U.S. Pat. No. 10,491,192, claims priority fromthe following provisional patent applications: application 62/685,825,filed Jun. 15, 2018, entitled SHEAR-MODE FBAR (XBAR); application62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODE FBAR (XBAR);application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZLATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883,filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR;and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUMTANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR.

Application Ser. No. 17/560,889 is also a continuation-in-part ofapplication Ser. No. 17/351,204, filed Jun. 17, 2021, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH REDUCED SPURIOUSMODES, which is a continuation of application Ser. No. 17/030,066, filedSep. 23, 2020, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATOR WITH REDUCED SPURIOUS MODES, now U.S. Pat. No. 11,146,232,which claims priority from provisional patent application 62/904,386,filed Sep. 23, 2019, entitled ANALYSIS OF XBAR RESONANCE AND HIGHERORDER SPURIOUS MODES.

Application Ser. No. 17/030,066 is also a continuation-in-part ofapplication Ser. No. 16/920,173, filed Jul. 2, 2020, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, now U.S. Pat. No.11,139,794, which is a continuation of application Ser. No. 16/438,121,filed Jun. 11, 2019, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATOR, now U.S. Pat. No. 10,756,697, which is a continuation-in-partof application Ser. No. 16/230,443, filed Dec. 21, 2018, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, now U.S. Pat. No.10,491,192 B2, which claims priority from the following provisionalpatent applications: application 62/685,825, filed Jun. 15, 2018,entitled SHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20,2018, entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filedOct. 5, 2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR(XBAR); application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODEFILM BULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR.

Application Ser. No. 17/560,889 is also a continuation-in-part ofapplication Ser. No. 17/169,155, filed Feb. 5, 2021, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH PERIODIC ETCHEDHOLES, which claims priority from provisional application 63/081,169,filed Sep. 21, 2020, entitled XBAR WITH HOLES and is a continuation inpart of application Ser. No. 16/930,534, filed Jul. 16, 2020, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH PERIODIC ETCHEDHOLES.

Application Ser. No. 16/930,534 claims priority from provisional patentapplication 62/874,709, filed Jul. 16, 2019, entitled XBAR WITH SLANTEDAND/OR PERFORATED MEMBRANE, and is also a continuation in part ofapplication Ser. No. 16/689,707, entitled BANDPASS FILTER WITH FREQUENCYSEPARATION BETWEEN SHUNT AND SERIES RESONATORS SET BY DIELECTRIC LAYERTHICKNESS, filed Nov. 20, 2019, now U.S. Pat. No. 10,917,070, which is acontinuation of application Ser. No. 16/230,443, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, filed Dec. 21, 2018,now U.S. Pat. No. 10,491,192, issued Nov. 26, 2019, which claimspriority from the following provisional patent applications: application62/685,825, filed Jun. 15, 2018, entitled SHEAR-MODE FBAR (XBAR);application 62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODE FBAR(XBAR); application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZLATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883,filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR,and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUMTANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR.

Application Ser. No. 17/560,889 is also a continuation-in-part ofapplication Ser. No. 16,989,710, filed Aug. 10, 2020, entitled FILTERUSING PIEZOELECTRIC FILM BONDED TO HIGH RESISTIVITY SILICON SUBSTRATEWITH TRAP-RICH LAYER, which claims priority to provisional patentapplication 62/951,452, filed Dec. 20, 2019, entitled PIEZOELECTRIC FILMBONDED TO HIGH RESISTIVITY SILICON HAVING TRAP-RICH LAYER FOR RFFILTERS. Application Ser. No. 16,989,710 is a continuation-in-part ofapplication Ser. No. 16/438,121, filed Jun. 11, 2019, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, now U.S. Pat. No.10,756,697, which is a continuation-in-part of application Ser. No.16/230,443, filed Dec. 21, 2018, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, now U.S. Pat. No. 10,491,192, which claims priorityfrom the following provisional patent applications: application62/685,825, filed Jun. 15, 2018, entitled SHEAR-MODE FBAR (XBAR);application 62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODE FBAR(XBAR); application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZLATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883,filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR;and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUMTANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR.

Application Ser. No. 17/560,889 is also a continuation-in-part ofapplication Ser. No. 17/115,765, filed Dec. 8, 2020, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH THERMALLYCONDUCTIVE ETCH-STOP LAYER, which is a continuation of application Ser.No. 16/933,224, titled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORWITH ETCH-STOP LAYER, filed Jul. 20, 2020, now U.S. Pat. No. 10,911,023,which claims priority to provisional patent application 62/978,133,titled XBAR WITH AL2O3 ETCH-STOP AND BONDING LAYER, filed Feb. 18, 2020,and provisional patent application 62/993,586, titled THIN FILM LAYER TOIMPROVE POWER OF XBAR RESONATORS, filed Mar. 23, 2020.

Application Ser. No. 17/560,889 is also a continuation-in-part ofapplication Ser. No. 17/217,923, filed Mar. 30, 2021, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH LOW THERMALIMPEDANCE, which claims priority to U.S. provisional patent applicationNo. 63/112,395, titled XBAR WITH LOW THERMAL IMPEDANCE, filed Nov. 11,2020.

Application Ser. No. 17/560,889 is a continuation-in-part of applicationSer. No. 17/229,767, filed Apr. 13, 2021, entitled SMALLTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS WITH ENHANCEDQ-FACTOR, which claim priority to the following provisional patentapplications: application No. 63/012,849, filed Apr. 20, 2020, entitledSMALL HIGH Q XBAR RESONATORS; application No. 63/066,520, filed Aug. 17,2020, entitled SMALL REFLECTORS TO IMPROVE XBAR LOSS; and applicationNo. 63/074,991, filed Sep. 4, 2020, entitled SMALL REFLECTORS TO IMPROVEPERFORMANCE OF TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS AT ASPECIFIED FREQUENCY.

Application Ser. No. 17/560,889 is also a continuation-in-part ofapplication Ser. No. 17/460,737, filed Aug. 20, 2021, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH INTERDIGITALTRANSDUCER WITH VARIED MARK AND PITCH, which is a continuation ofapplication Ser. No. 16/805,471, filed Feb. 28, 2020, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH INTERDIGITALTRANSDUCER WITH VARIED MARK AND PITCH, which claims priority fromprovisional patent application 62/892,871, filed Aug. 28, 2019, entitledXBAR RESONATORS WITH REDUCED SPURIOUS MODES.

Application Ser. No. 17/560,889 is also a continuation-in-part ofapplication Ser. No. 17/388,745, filed Jul. 29, 2021, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH MULTI-MARKINTERDIGITAL TRANSDUCER, which claims priority from provisional patentapplication 63/144,977, filed Feb. 3, 2021, entitled CHIRPED XBARELECTRODES.

Application Ser. No. 17/560,889 is also a continuation-in-part ofapplication Ser. No. 17/122,977, Dec. 15, 2020, entitled ACOUSTICRESONATORS AND FILTERS WITH REDUCED TEMPERATURE COEFFICIENT OFFREQUENCY, which claims priority to provisional patent application63/053,584, filed Jul. 18, 2020, entitled TCF OPTIMIZED XBAR DEVICES,and provisional patent application 63/088,344, filed Oct. 6, 2020,entitled OPTIMAL CUT ANGLE TO REDUCE TCF OF XBAR FILTERS.

All of these applications are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigitaltransducer (IDT) formed on a thin floating layer, or diaphragm, of asingle-crystal piezoelectric material. The IDT includes a first set ofparallel fingers, extending from a first busbar and a second set ofparallel fingers extending from a second busbar. The first and secondsets of parallel fingers are interleaved. A microwave signal applied tothe IDT excites a shear primary acoustic wave in the piezoelectricdiaphragm. XBAR resonators provide very high electromechanical couplingand high frequency capability. XBAR resonators may be used in a varietyof RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are well suited for use in filtersfor communications bands with frequencies above 3 GHz. Matrix XBARfilters are also suited for frequencies between 1 GHz and 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectionalviews, and a detail view of a transversely-excited film bulk acousticresonator (XBAR).

FIG. 2A is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 2B is another alternative schematic cross-sectional view of theXBAR of FIG. 1.

FIG. 2C is an alternative schematic plan view of an XBAR

FIG. 3 is a graphic illustrating a shear primary acoustic mode in anXBAR.

FIG. 4 is a schematic circuit diagram of a band-pass filter usingacoustic resonators in a ladder circuit.

FIG. 5 is a graph showing the relationship between piezoelectricdiaphragm thickness and resonance frequency of an XBAR.

FIG. 6 is a plot showing the relationship between coupling factor Gamma(F) and IDT pitch for an XBAR.

FIG. 7 is a graph showing the dimensions of XBAR resonators withcapacitance equal to one picofarad.

FIG. 8 is a graph showing the relationship between IDT finger pitch andresonance and anti-resonance frequencies of an XBAR, with dielectriclayer thickness as a parameter.

FIG. 9 is a graph identifying preferred combinations of aluminum IDTthickness and IDT pitch for XBARs without a front dielectric layer.

FIG. 10 is a graph identifying preferred combinations of aluminum IDTthickness and IDT pitch for XBARs with front dielectric layer thicknessequal to 0.25 times the XBAR diaphragm thickness.

FIG. 11 is a graph identifying preferred combinations of copper IDTthickness and

IDT pitch for XBARs without a front dielectric layer.

FIG. 12 is a graph identifying preferred combinations of copper IDTthickness and IDT pitch for XBARs with front dielectric layer thicknessequal to 0.25 times the XBAR diaphragm thickness.

FIG. 13 is a graph identifying preferred combinations of aluminum IDTthickness and IDT pitch for XBARs without a front dielectric layer fordiaphragm thicknesses of 300 nm, 400 nm, and 500 nm.

FIG. 14 is a graph of the frequency and coupling of a primary shear modeand the frequency of an A1-3 spurious mode as functions of interdigitaltransducer (IDT) pitch.

FIG. 15 is a graph of the frequency and coupling of the primary shearmode as functions of IDT mark-to-pitch ratio.

FIG. 16 is a graph of absolute admittance and frequency of the A1-3spurious mode as functions of IDT mark-to-pitch ratio.

FIG. 17 is an expanded schematic plan view of a unit cell for aninterdigital transducer (IDT) with pitch and mark varied along thelength of the IDT.

FIG. 18 is a schematic plan view of an IDT made up of eight of the unitcells of FIG. 5.

FIG. 19 is a plan view of a multi-mark interdigital transducer (IDT).

FIG. 20 is a plan view of another multi-mark IDT.

FIG. 21 is a graphical representation of Euler angles.

FIG. 22 is a chart of the e14 and e15 piezoelectric coefficients of alithium niobate plate with Euler angles [0°, β, 0° ] as functions of β.

FIG. 23 is a graph of piezoelectric coupling and temperature coefficientof velocity as functions of Euler angle β for lithium niobate.

FIG. 24 is a chart of the electromechanical coupling of an XBAR as afunction of Z-axis tilt angle β.

FIG. 25 is a schematic plan view of an IDT with reflector elements.

FIG. 26A is a schematic cross-sectional view of an XBAR with anetch-stop layer and back-side etched cavities.

FIG. 26B is a schematic cross-sectional view of an XBAR with anetch-stop layer and front-side etched cavities.

FIG. 27 is schematic cross-sectional view and two detailedcross-sectional views of an XBAR.

FIG. 28 is a plan view and a cross-sectional view of an XBAR withperiodic etched holes.

FIG. 29A is a schematic cross-sectional view of XBAR device havingpredetermined areas of the bonding layer and piezoelectric layer removedfrom selected locations.

FIG. 29B is a schematic cross-sectional view of XBAR device having threepredetermined areas of the bonding layer and piezoelectric layer removedfrom selected locations.

FIG. 30 is a flow chart of a process for fabricating an XBAR device.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION Description of Apparatus

FIG. 1 shows a simplified schematic top view, orthogonal cross-sectionalviews, and a detailed cross-sectional view of a transversely-excitedfilm bulk acoustic resonator (XBAR) 100. XBAR resonators such as theresonator 100 may be used in a variety of RF filters includingband-reject filters, band-pass filters, duplexers, and multiplexers.XBARs are particularly suited for use in filters for communicationsbands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the front and back surfaces 112, 114.However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations. A thickness between the front and backsurfaces 112, 114 of the piezoelectric plate can be greater than orequal to 200 nm and less than or equal to 1000 nm.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”. In other configurations, the diaphragm 115 may be contiguous withthe piezoelectric plate around at least 50% of the perimeter 145 of thecavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 under the diaphragm 115. The cavity 140 may be formed, for example,by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode (e.g., ashear primary acoustic mode) within the piezoelectric plate 110. As willbe discussed in further detail, the primary acoustic mode is a bulkshear mode where acoustic energy propagates along a directionsubstantially orthogonal to the surface of the piezoelectric plate 110,which is also normal, or transverse, to the direction of the electricfield created by the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate which spans, or is suspended over, the cavity140. As shown in FIG. 1, the cavity 140 has a rectangular shape with anextent greater than the aperture AP and length L of the IDT 130. Acavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may have more or fewer thanfour sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds ofparallel fingers in the IDT 110. Similarly, the thickness of the fingersin the cross-sectional views is greatly exaggerated.

Referring to the detailed cross-sectional view, a front-side dielectriclayer 150 may optionally be formed on the front side of thepiezoelectric plate 110. The “front side” of the XBAR is, by definition,the surface facing away from the substrate. The front-side dielectriclayer 150 may be formed only between the IDT fingers (e.g. IDT finger138 b) or may be deposited as a blanket layer such that the dielectriclayer is formed both between and over the IDT fingers (e.g. IDT finger138 a). The front-side dielectric layer 150 may be a non-piezoelectricdielectric material, such as silicon dioxide or silicon nitride. Thethickness of the front side dielectric layer is typically less than orequal to the thickness of the piezoelectric plate. The front-sidedielectric layer 150 may be formed of multiple layers of two or morematerials. A resonant frequency of the XBAR may be determined in part bya thickness of the front-side dielectric layer.

The IDT fingers 138 a and 138 b may be aluminum, an aluminum alloy,copper, a copper alloy, beryllium, gold, tungsten, molybdenum or someother conductive material. The IDT fingers are considered to be“substantially aluminum” if they are formed from aluminum or an alloycomprising at least 50% aluminum. The IDT fingers are considered to be“substantially copper” if they are formed from copper or an alloycomprising at least 50% copper. Thin (relative to the total thickness ofthe conductors) layers of other metals, such as chromium or titanium,may be formed under and/or over and/or as layers within the fingers toimprove adhesion between the fingers and the piezoelectric plate 110and/or to passivate or encapsulate the fingers and/or to improve powerhandling. The busbars 132, 134 of the IDT may be made of the same ordifferent materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers.

FIG. 2A and FIG. 2B show two alternative cross-sectional views along thesection plane A-A defined in FIG. 1. In FIG. 2A, a piezoelectric plate210 is attached to a substrate 220. A portion of the piezoelectric plate210 forms a diaphragm 215 spanning a cavity 240 in the substrate. Thecavity 240 does not fully penetrate the substrate 220. Fingers of an IDTare disposed on the diaphragm 215. The cavity 240 may be formed, forexample, by etching the substrate 220 before attaching the piezoelectricplate 210. Alternatively, the cavity 240 may be formed by etching thesubstrate 220 with a selective etchant that reaches the substratethrough one or more openings (not shown) provided in the piezoelectricplate 210. In this case, the diaphragm 215 may be contiguous with therest of the piezoelectric plate 210 around a large portion of aperimeter 245 of the cavity 240. For example, the diaphragm 215 may becontiguous with the rest of the piezoelectric plate 210 around at least50% of the perimeter 245 of the cavity 240. A intermediate layer (notshown), such as a dielectric bonding layer, may be present between thepiezo electric plate 240 and the substrate 220.

In FIG. 2B, the substrate 220 includes a base 222 and an intermediatelayer 224 disposed between the piezoelectric plate 210 and the base 222.For example, the base 222 may be silicon and the intermediate layer 224may be silicon dioxide or silicon nitride or some other material. Aportion of the piezoelectric plate 210 forms a diaphragm 215 spanning acavity 240 in the intermediate layer 224. Fingers of an IDT are disposedon the diaphragm 215. The cavity 240 may be formed, for example, byetching the intermediate layer 224 before attaching the piezoelectricplate 210. Alternatively, the cavity 240 may be formed by etching theintermediate layer 224 with a selective etchant that reaches thesubstrate through one or more openings provided in the piezoelectricplate 210. In this case, the diaphragm 215 may be contiguous with therest of the piezoelectric plate 210 around a large portion of aperimeter 245 of the cavity 240. For example, the diaphragm 215 may becontiguous with the rest of the piezoelectric plate 210 around at least50% of the perimeter 245 of the cavity 240 as shown in FIG. 2C. Althoughnot shown in FIG. 2B, a cavity formed in the intermediate layer 224 mayextend into the base 222.

FIG. 2C is a schematic plan view of another XBAR 250. The XBAR 250includes an IDT formed on a piezoelectric plate 210. A portion of thepiezoelectric plate 210 forms a diaphragm spanning a cavity in asubstrate. In this example, the perimeter 245 of the cavity has anirregular polygon shape such that none of the edges of the cavity areparallel, nor are they parallel to the conductors of the IDT. A cavitymay have a different shape with straight or curved edges.

FIG. 3 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 3 shows a small portion of an XBAR 300including a piezoelectric plate 310 and three interleaved IDT fingers330 which alternate in electrical polarity from finger to finger. An RFvoltage is applied to the interleaved fingers 330. This voltage createsa time-varying electric field between the fingers. The direction of theelectric field is predominantly lateral, or parallel to the surface ofthe piezoelectric plate 310, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the RF electric energy is highly concentratedinside the plate relative to the air. The lateral electric fieldintroduces shear deformation which couples strongly to a shear primaryacoustic mode (at a resonance frequency defined by the acoustic cavityformed by the volume between the two surfaces of the piezoelectricplate) in the piezoelectric plate 310. In this context, “sheardeformation” is defined as deformation in which parallel planes in amaterial remain predominantly parallel and maintain constant separationwhile translating (within their respective planes) relative to eachother. A “shear acoustic mode” is defined as an acoustic vibration modein a medium that results in shear deformation of the medium. The sheardeformations in the XBAR 300 are represented by the curves 360, with theadjacent small arrows providing a schematic indication of the directionand relative magnitude of atomic motion at the resonance frequency. Thedegree of atomic motion, as well as the thickness of the piezoelectricplate 310, have been greatly exaggerated for ease of visualization.While the atomic motions are predominantly lateral (i.e. horizontal asshown in FIG. 3), the direction of acoustic energy flow of the excitedprimary acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 365.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 4 is a schematic circuit diagram of a band-pass filter 400 usingfive XBARs X1-X5. The filter 400 may be, for example, a band n79band-pass filter for use in a communication device. The filter 400 has aconventional ladder filter architecture including three seriesresonators X1, X3, X5 and two shunt resonators X2, X4. The three seriesresonators X1, X3, X5 are connected in series between a first port and asecond port. In FIG. 4, the first and second ports are labeled “In” and“Out”, respectively. However, the filter 400 is symmetrical and eitherport may serve as the input or output of the filter. The two shuntresonators X2, X4 are connected from nodes between the series resonatorsto ground. All the shunt resonators and series resonators are XBARs.

The three series resonators X1, X3, X5 and the two shunt resonators X2,X4 of the filter 400 may be formed on a single plate 430 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown), with at least thefingers of the IDT disposed over a cavity in the substrate. In this andsimilar contexts, the term “respective” means “relating things each toeach”, which is to say with a one-to-one correspondence. In FIG. 4, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 435). In this example, an IDT of each resonator isdisposed over a respective cavity. In other filters, the IDTs of two ormore resonators may be disposed over a common cavity. Resonators mayalso be cascaded into multiple IDTs which may be formed on multiplecavities.

Each of the resonators X1 to X5 has a resonance frequency and ananti-resonance frequency. In over-simplified terms, each resonator iseffectively a short circuit at its resonance frequency and effectivelyan open circuit at its anti-resonance frequency. Each resonator X1 to X5creates a “transmission zero”, where the transmission between the in andout ports of the filter is very low. Note that the transmission at a“transmission zero” is not actually zero due to energy leakage throughparasitic components and other effects. The three series resonators X1,X3, X5 create transmission zeros at their respective anti-resonancefrequencies (where each resonator is effectively an open circuit). Thetwo shunt resonators X2, X4 create transmission zeros at theirrespective resonance frequencies (where each resonator is effectively ashort circuit). In a typical band-pass filter using acoustic resonators,the anti-resonance frequencies of the series resonators are above thepassband, and the resonance frequencies of the shunt resonators arebelow the passband.

A band-pass filter for use in a communications device, such as acellular telephone, must meet a variety of requirements. First, aband-pass filter, by definition, must pass, or transmit with acceptableloss, a defined pass-band. Typically, a band-pass filter for use in acommunications device must also stop, or substantially attenuate, one ormore stop band(s). For example, a band n79 band-pass filter is typicallyrequired to pass the n79 frequency band from 4400 MHz to 5000 MHz and tostop the 5 GHz WiFi™ band and/or the n77 band from 3300 MHz to 4200 MHz.To meet these requirements, a filter using a ladder circuit wouldrequire series resonators with anti-resonance frequencies about or above5100 MHz, and shunt resonators with resonance frequencies about or below4300 MHz.

The resonance and anti-resonance frequencies of an XBAR are stronglydependent on the thickness ts of the piezoelectric membrane (115 in FIG.1). FIG. 5 is a graph 500 of resonance frequency of an XBAR versuspiezoelectric diaphragm thickness. In this example, the piezoelectricdiaphragm is z-cut lithium niobate. The solid curve 510 is plot ofresonance frequency as function of the inverse of the piezoelectricplate thickness for XBARs with IDT pitch equal to 3 microns. This plotis based on results of simulations of XBARs using finite elementmethods. The resonance frequency is roughly proportional to the inverseof the piezoelectric plate thickness.

The resonance and anti-resonance frequencies of an XBAR are alsodependent on the pitch (dimension p in FIG. 1) of the IDT. Further, theelectromechanical coupling of an XBAR, which determines the separationbetween the resonance and anti-resonance frequencies, is dependent onthe pitch. FIG. 6 is a graph of gamma (F) as a function of normalizedpitch, which is to say IDT pitch p divided by diaphragm thickness ts.Gamma is a metric defined by the equation:

$\Gamma = \frac{1}{\left( {{Fa}/{Fr}} \right)^{2} - 1}$

where Fa is the antiresonance frequency and Fr is the resonancefrequency. Large values for gamma correspond to smaller separationbetween the resonance and anti-resonance frequencies. Low values ofgamma indicate strong coupling which is good for wideband ladderfilters.

In this example, the piezoelectric diaphragm is z-cut lithium niobate,and data is presented for diaphragm thicknesses of 300 nm, 400 nm, and500 nm. In all cases the IDT is aluminum with a thickness of 25% of thediaphragm thickness, the duty factor (i.e. the ratio of the width w tothe pitch p) of the IDT fingers is 0.14, and there are no dielectriclayers. The “+” symbols, circles, and “x” symbols represent diaphragmthicknesses of 300 nm, 400 nm, and 500 nm, respectively. Outlier datapoints, such as those for relative IDT pitch about 4.5 and about 8, arecaused by spurious modes interacting with the primary acoustic mode andaltering the apparent gamma. The relationship between gamma and IDTpitch is relatively independent of diaphragm thickness, and roughlyasymptotic to Γ=3.5 as the relative pitch is increased.

Another typical requirement on a band-pass filter for use in acommunications device is the input and output impedances of the filterhave to match, at least over the pass-band of the filter, the impedancesof other elements of the communications device to which the filter isconnected (e.g. a transmitter, receiver, and/or antenna) for maximumpower transfer. Commonly, the input and output impedances of a band-passfilter are required to match a 50-ohm impedance within a tolerance thatmay be expressed, for example, as a maximum return loss or a maximumvoltage standing wave ratio. When necessary, an impedance matchingnetwork comprising one or more reactive components can be used at theinput and/or output of a band-pass filter. Such impedance matchingnetworks add to the complexity, cost, and insertion loss of the filterand are thus undesirable. To match, without additional impedancematching components, a 50-Ohm impedance at a frequency of 5 GHz, thecapacitances of at least the shunt resonators in the band-pass filterneed to be in a range of about 0.5 picofarads (pF) to about 1.5picofarads.

FIG. 7 is a graph showing the area and dimensions of XBAR resonatorswith capacitance equal to one picofarad. The solid line 710 is a plot ofthe IDT length required provide a capacitance of 1 pF as a function ofthe inverse of the IDT aperture when the IDT pitch is 3 microns. Thedashed line 720 is a plot of the IDT length required provide acapacitance of 1 pF as a function of the inverse of the IDT aperturewhen the IDT pitch is 5 microns. The data plotted in FIG. 7 is specificto XBAR devices with lithium niobate diaphragm thickness of 400 nm.

For any aperture, the IDT length required to provide a desiredcapacitance is greater for an IDT pitch of 5 microns than for an IDTpitch of 3 microns. The required IDT length is roughly proportional tothe change in IDT pitch. The design of a filter using XBARs is acompromise between somewhat conflicting objectives. As shown in FIG. 6,a larger IDT pitch may be preferred to reduce gamma and maximize theseparation between the anti-resonance and resonance frequencies. As canbe understood from FIG. 7, smaller IDT pitch is preferred to minimizeIDT area. A reasonable compromise between these objectives is6≤p/ts≤12.5. Setting the IDT pitch p equal to or greater than six timesthe diaphragm thickness ts provides Fa/Fr greater than 1.1. Setting themaximum IDT pitch p to 12.5 times the diaphragm thickness ts isreasonable since Fa/Fr does not increase appreciably for higher valuesof relative pitch.

As will be discussed is greater detail subsequently, the metal fingersof the IDTs provide the primary mechanism for removing heat from an XBARresonator. Increasing the aperture of a resonator increases the lengthand the electrical and thermal resistance of each IDT finger. Further,for a given IDT capacitance, increasing the aperture reduces the numberof fingers required in the IDT, which, in turn, proportionally increasesthe RF current flowing in each finger. All of these effects argue forusing the smallest possible aperture in resonators for high-powerfilters.

Conversely, several factors argue for using a large aperture. First, thetotal area of an XBAR resonator includes the area of the IDT and thearea of the bus bars. The area of the bus bars is generally proportionalto the length of the IDT. For very small apertures, the area of the IDTbus bars may be larger than the area occupied by the interleaved IDTfingers. Further, some electrical and acoustic energy may be lost at theends of the IDT fingers. These loss effects become more significant asIDT aperture is reduced and the total number of fingers is increased.These losses may be evident as a reduction in resonator Q-factor,particularly at the anti-resonance frequency, as IDT aperture isreduced.

As a compromise between conflicting objectives, resonators apertureswill typically fall in the range from 20 μm and 60 μm.

The resonance and anti-resonance frequencies of an XBAR are alsodependent on the thickness (dimension tfd in FIG. 2) of the front-sidedielectric layer applied between (and optionally over) the fingers ofthe IDT. FIG. 8 is a graph 800 of anti-resonant frequency and resonantfrequency as a function of IDT finger pitch p for XBAR resonators withz-cut lithium niobate piezoelectric plate thickness ts=400 nm, withfront-side dielectric layer thickness tfd as a parameter. The solidlines 810 and 820 are plots of the anti-resonance and resonancefrequencies, respectively, as functions of IDT pitch for tfd=0. Thedashed lines 812 and 822 are plots of the anti-resonance and resonancefrequencies, respectively, as functions of IDT pitch for tfd=30 nm. Thedash-dot lines 814 and 824 are plots of the anti-resonance and resonancefrequencies, respectively, as functions of IDT pitch for tfd=60 nm. Thedash-dot-dot lines 816 and 826 are plots of the anti-resonance andresonance frequencies, respectively, as functions of IDT pitch fortfd=90 nm. The frequency shifts are approximately linear functions oftfd.

In FIG. 8, the difference between the resonance and anti-resonancefrequencies is 600 to 650 MHz for any particular values for front-sidedielectric layer thickness and IDT pitch. This difference is largecompared to that of older acoustic filter technologies, such as surfaceacoustic wave filters. However, 650 MHz is not sufficient for very wideband filters such as band-pass filters needed for bands n77 and n79. Asdescribed in application Ser. No. 16/230,443, the front-side dielectriclayer over shunt resonators may be thicker than the front-sidedielectric layer over series resonators to increase the frequencydifference between the resonant frequencies of the shunt resonators andthe anti-resonance frequencies of the series resonators.

Communications devices operating in time-domain duplex (TDD) bandstransmit and receive in the same frequency band. Both the transmit andreceive signal paths pass through a common bandpass filter connectedbetween an antenna and a transceiver. Communications devices operatingin frequency-domain duplex (FDD) bands transmit and receive in differentfrequency bands. The transmit and receive signal paths pass throughseparate transmit and receive bandpass filters connected between anantenna and the transceiver. Filters for use in TDD bands or filters foruse as transmit filters in FDD bands can be subjected to radio frequencyinput power levels of 30 dBm or greater and must avoid damage underpower.

The insertions loss of acoustic wave bandpass filters is usually notmore than a few dB. Some portion of this lost power is return lossreflected back to the power source; the rest of the lost power isdissipated in the filter. Typical band-pass filters for LTE bands havesurface areas of 1.0 to 2.0 square millimeters. Although the total powerdissipation in a filter may be small, the power density can be highgiven the small surface area. Further, the primary loss mechanisms in anacoustic filter are resistive losses in the conductor patterns andacoustic losses in the IDT fingers and piezoelectric material. Thus thepower dissipation in an acoustic filter is concentrated in the acousticresonators. To prevent excessive temperature increase in the acousticresonators, the heat due to the power dissipation must be conducted awayfrom the resonators through the filter package to the environmentexternal to the filter.

In traditional acoustic filters, such as surface acoustic wave (SAW)filters and bulk acoustic wave (BAW) filters, the heat generated bypower dissipation in the acoustic resonators is efficiently conductedthrough the filter substrate and the metal electrode patterns to thepackage. In an XBAR device, the resonators are disposed on thinpiezoelectric membranes that are inefficient heat conductors. The largemajority of the heat generated in an XBAR device must be removed fromthe resonator via the IDT fingers and associated conductor patterns.

To minimize power dissipation and maximize heat removal, the IDT fingersand associated conductors should be formed from a material that has lowelectrical resistivity and high thermal conductivity. Metals having bothlow resistivity and high thermal conductivity are listed in thefollowing table:

Electrical Thermal resistivity conductivity Metal (10⁻⁶ Ω-cm) (W/m-K)Silver 1.55 419 Copper 1.70 385 Gold 2.2 301 Aluminium 2.7 210

Silver offers the lowest resistivity and highest thermal conductivitybut is not a viable candidate for IDT conductors due to the lack ofprocesses for deposition and patterning of silver thin films.Appropriate processes are available for copper, gold, and aluminum.Aluminum offers the most mature processes for use in acoustic resonatordevices and potentially the lowest cost, but with higher resistivity andreduced thermal conductivity compared to copper and gold. Forcomparison, the thermal conductivity of lithium niobate is about 4W/m-K, or about 2% of the thermal conductivity of aluminum. Aluminumalso has good acoustic attenuation properties which helps minimizedissipation.

The electric resistance of the IDT fingers can be reduced, and thethermal conductivity of the IDT fingers can be increased, by increasingthe cross-sectional area of the fingers to the extent possible. Asdescribed in conjunction with FIG. 3, unlike SAW or AN BAW, for XBARthere is little coupling of the primary acoustic mode to the IDTfingers. Changing the width and/or thickness of the IDT fingers hasminimal effect on the primary acoustic mode in an XBAR device. This is avery uncommon situation for an acoustic wave resonator. However, the IDTfinger geometry does have a substantial effect on coupling to spuriousacoustic modes, such as higher order shear modes and plate modes thattravel laterally in the piezoelectric diaphragm.

Given the complex dependence of spurious mode frequency and amplitude ondiaphragm thickness ts, IDT metal thickness tm, IDT pitch p and IDTfinger width w, the inventors undertook an empirical evaluation, usingtwo-dimensional finite element modeling, of a large number ofhypothetical XBAR resonators. For each combination of diaphragmthickness ts, IDT finger thickness tm, and IDT pitch p, the XBARresonator was simulated for a sequence of IDT finger width w values. Afigure of merit (FOM) was calculated for each value if w to estimate thenegative impact of spurious modes. The FOM is calculated by integratingthe negative impact of spurious modes across a defined frequency range.The FOM and the frequency range depend on the requirements of aparticular filter. The frequency range typically includes the passbandof the filter and may include one or more stop bands. Spurious modesoccurring between the resonance and anti-resonance frequencies of eachhypothetical resonator were given a heavier weight in the FOM thanspurious modes at frequencies below resonance or above anti-resonance.Hypothetical resonators having a minimized FOM below a threshold valuewere considered potentially “useable”, which is to say probably havingsufficiently low spurious modes for use in a filter. Hypotheticalresonators having a minimized cost function above the threshold valuewere considered not useable.

FIG. 9 is a chart 900 showing combinations of IDT pitch and IDT fingerthickness that may provide useable resonators. This chart is based ontwo-dimensional simulations of XBARs with lithium niobate diaphragmthickness ts=400 nm, aluminum conductors, and front-side dielectricthickness tfd=0. XBARs with IDT pitch and thickness within shadedregions 910, 915, 920, 930 are likely to have sufficiently low spuriouseffects for use in filters. For each combination of IDT pitch and IDTfinger thickness, the width of the IDT fingers was selected to minimizethe FOM. Usable resonators exist for IDT finger thickness greater thanor equal to 340 nm and less than or equal to 1000 nm.

As previously discussed, wide bandwidth filters using XBARs may use athicker front-side dielectric layer on shunt resonators than on seriesresonators to lower the resonance frequencies of the shunt resonatorswith respect to the resonance frequencies of the series resonators. Thefront-side dielectric layer on shunt resonators may be as much as 150 nmthicker than the front side dielectric on series resonators. For ease ofmanufacturing, it may be preferable that the same IDT finger thicknessbe used on both shunt and series resonators.

FIG. 10 is another chart 1000 showing combinations of IDT pitch and IDTfinger thickness that may provide useable resonators. This chart isbased on simulations of XBARs with lithium niobate diaphragmthickness=400 nm, aluminum conductors, and tfd=100 nm. XBARs having IDTpitch and thickness within shaded regions 1010, 1020, 1030 are likely tohave sufficiently low spurious effects for use in filters. For eachcombination of IDT pitch and IDT finger thickness, the width of the IDTfingers was selected to minimize the FOM. Usable resonators exist forIDT finger thickness greater than or equal to 350 nm and less than orequal to 900 nm.

Assuming that a filter is designed with no front-side dielectric layeron series resonators and 100 nm of front-side dielectric on shuntresonators, FIG. 9 and FIG. 10 jointly define the combinations of metalthickness and IDT pitch that result in useable resonators. Specifically,FIG. 9 defines useable combinations of metal thickness and IDT pitch forseries resonators and FIG. 10 defines useable combinations of metalthickness and IDT for shunt resonators. Since only a single metalthickness is desirable for ease of manufacturing, the overlap betweenthe ranges defined in FIG. 9 and FIG. 10 defines the range of metalthicknesses for filter using a front-side dielectric to shift theresonance frequency of shunt resonator. Comparing FIG. 9 and FIG. 10,IDT aluminum thickness between 350 nm and 900 nm (350 nm≤tm≤900 nm)provides at least one useable value of pitch for both series and shuntresonators.

FIG. 11 is another chart 1100 showing combinations of IDT pitch and IDTfinger thickness that may provide useable resonators. The chart iscomparable to FIG. 9 with copper, rather than aluminum, conductors. FIG.11 is based on simulations of XBARs with lithium niobate diaphragmthickness=400 nm, copper conductors, and tfd=0. XBARs having IDT pitchand finger width within shaded regions 1110, 1120, 1130, 1140 are likelyto have sufficiently low spurious effects for use in filters. For eachcombination of IDT pitch and IDT finger thickness, the width of the IDTfingers is selected to minimize the FOM. Usable resonators exist for IDTfinger thickness greater than or equal to 340 nm and less than or equalto 570 nm, and for IDT finger thickness greater than or equal to 780 nmand less than or equal to 930 nm.

FIG. 12 is another chart 1200 showing combinations of IDT pitch and IDTfinger thickness that may provide usable resonators. This chart is basedon simulations of XBARs with lithium niobate diaphragm thickness=400 nm,copper conductors, and tfd=100 nm. XBARs having IDT pitch and fingerthickness within shaded regions 1210, 1220 are likely to havesufficiently low spurious effects for use in filters. For eachcombination of IDT pitch and IDT finger thickness, the width of the IDTfingers is selected to minimize the cost function. IDT finger thicknessgreater than or equal to 340 nm and less than or equal to 770 nm

Assuming that a filter is designed with no front-side dielectric layeron series resonators and 100 nm of front-side dielectric on shuntresonators, FIG. 11 and FIG. 12 jointly define the combinations of metalthickness and IDT pitch that result in useable resonators. Specifically,FIG. 11 defines useful combinations of metal thickness and IDT pitch forseries resonators and FIG. 12 defines useful combinations of metalthickness and IDT pitch for shunt resonators. Since only a single metalthickness is desirable for ease of manufacturing, the overlap betweenthe ranges defined in FIG. 11 and FIG. 12 defines the range of metalthicknesses for filter using a front-side dielectric to shift theresonance frequency of shunt resonator. Comparing FIG. 11 and FIG. 12,IDT copper thickness between 340 nm and 570 nm provides at least oneuseable value of pitch for series and shunt resonators.

Charts similar to FIG. 9, FIG. 10, FIG. 11, and FIG. 12, can be preparedfor other values of front-side dielectric thickness, and other conductormaterials such as Gold.

FIG. 13 is a chart 1300 showing combinations of IDT pitch and IDT fingerthickness that may provide useable resonators on different thicknessdiaphragms. The shaded regions 1310, 1315, 1320 define useablecombinations of IDT pitch and aluminum IDT thickness for a diaphragmthickness of 500 nm. The areas enclosed by solid lines, such as line1330, define useable combinations of IDT pitch and aluminum IDTthickness for a diaphragm thickness of 400 nm. The solid lines are theboundaries of the shaded areas 910, 915, and 920 of FIG. 9. The areasenclosed by dashed lines, such as line 1340, define useable combinationsof IDT pitch and aluminum IDT thickness for a diaphragm thickness of 300nm.

Although the combinations of IDT thickness and pitch that result inuseable resonators on 500 nm diaphragms (shaded regions 1310, 1315,1320), 400 nm diaphragms (regions enclosed by solid lines), and 300 nmdiaphragms (regions enclosed by dashed lines) are not identical, thesame general trends are evident. For diaphragm thicknesses of 300, 400,and 500 nm, useable resonators may be made with IDT metal thickness lessthan about 0.375 times the diaphragm thickness. Further, for diaphragmthicknesses of 300, 400, and 500 nm, useable resonators may be made withIDT aluminum thickness greater than about 0.85 times the diaphragmthickness and up to at least 1.5 times the diaphragm thickness. Althoughnot shown in FIG. 13, it is believed that the conclusions drawn fromFIG. 9 to FIG. 12 can be scaled with diaphragm thickness. For aluminumIDT conductors, the range of IDT thickness that will provide usefulresonators is given by the formula 0.85≤tm/ts≤2.5. For filters using afront-side dielectric to shift the resonance frequency of shuntresonators, the range of aluminum IDT thickness that will provide usefulresonators is given by the formula 0.875≤tm/ts≤2.25. For copper IDTconductors, the range of IDT thickness that will provide usefulresonators is given by the formula 0.85≤tm/ts≤1.42 or the formula1.95≤tm/ts≤2.325. For filters using a front-side dielectric to shift theresonance frequency of shunt resonators, the range of aluminum IDTthickness that will provide useful resonators is given by the formula0.85≤tm/ts≤1.42.

Experimental results indicate that thin IDT fingers (i.e. tm/ts<0.375)cannot adequately transport heat out of the resonator area and IDTs withsuch thin IDT fingers are unsuitable for high power applications. ThickIDT conductors (i.e. tm/ts>0.85) provide greatly improved heattransport. Experimental results indicate that filters using XBARresonators with 500 nm aluminum IDT fingers and 400 nm diaphragmthickness (tm/ts=1.25) can tolerate 31 dBm CW (continuous wave) RF powerinput at the upper edge of the filter passband (commonly the frequencywith the highest power dissipation within a filter passband).

FIG. 14 is a graph 1400 of relationships between XBAR performance andIDT pitch. The solid curve 1410 is a plot of the resonance frequency ofthe XBAR shear primary mode as a function of IDT pitch for an XBAR witha z-cut lithium niobate diaphragm 400 nm thick and aluminum conductors100 nm thick. The solid curve 1410 is read against the left verticalaxis of the graph. The diaphragm thickness is the dominant parameterthat determines resonance frequency of an XBAR. The resonance frequencyhas a smaller dependence on IDT pitch. As shown in FIG. 14, varying theIDT pitch from 1 μm to 6 μm results in reduction in resonance frequencyby about 25%.

The dashed curve 1420 is a plot of the resonance frequency of the A1-3spurious mode of the same XBAR as a function of IDT pitch. The dashedcurve 1420 is read against the left vertical axis of the graph.Diaphragm thickness is also the dominant parameter that determinesresonance frequency of A1-3 mode. However, the resonance frequency ofthe A1-3 mode has a much larger dependence on IDT pitch compared to theshear primary mode. Varying the IDT pitch from 1 μm to 6 μm results inreduction in resonance frequency of the A1-4 mode by about 85%. Thefrequencies of other spurious modes (i.e. spurious modes 640 in FIG. 6)are also very dependent on IDT pitch.

The dot-dash curve 1430 is a plot of electromechanical coupling of theshear primary mode as a function of IDT pitch. The dot-dash curve 1430is read against the right vertical axis of the graph. The relationshipbetween coupling and pitch is nonlinear. Larger IDT pitch results inhigher coupling and coupling decreases rapidly for pitch values lessthan 3 μm. However, 17% coupling is available at pitch value of 1 μm,which is still sufficient for many filter applications.

FIG. 15 is a graph 1500 of relationships between XBAR performance andIDT mark-to-pitch ratio (mark/pitch). The solid curve 1510 is a plot ofthe resonance frequency of the XBAR shear primary mode as a function ofIDT mark/pitch for an XBAR with a z-cut lithium niobate diaphragm 400 nmthick. The solid curve 1510 is read against the left vertical axis ofthe graph. The IDT conductors are aluminum 100 nm thick and the IDTpitch is 3 μm. The diaphragm thickness is the dominant parameter thatdetermines resonance frequency of an XBAR. The resonance frequency has asmaller dependence on IDT mark/pitch. As shown in FIG. 15, varying theIDT mark/pitch from 0.15 to 0.45 μm results in reduction in resonancefrequency by about 6%.

The dashed curve 1520 is a plot of electromechanical coupling of theshear primary mode as a function of IDT mark/pitch. The dashed curve1520 is read against the right vertical axis of the graph. Therelationship between coupling and mark/pitch is nonlinear. Maximumcoupling occurs for IDT mark/pitch between 0.40 and 0.45. Couplingdecreases with decreasing mark/pitch. However, 27% coupling is availableat mark/pitch value of about 0.12, which is sufficient for most filterapplications.

FIG. 14 and FIG. 15 illustrate the complexity of selecting the pitch andmark of XBAR IDTs within a filter to provide a desired resonancefrequency and electromechanical coupling of each XBAR while trying toplace spurious modes at frequencies that do not degrade the filterperformance. In particular, since the resonance frequency of the shearprimary acoustic mode varies with both IDT pitch and IDT mark, the pitchand mark must be selected in combination to set the resonance frequencyof an XBAR to a predetermined target frequency. Since the same resonancefrequency may be achieved with different IDT pitch and markcombinations, a filter designer has some freedom to select the pitch andmark to control the frequencies of spurious modes.

FIG. 16 is a graph 1600 of relationships between the A1-3 spurious modeand IDT mark-to-pitch ratio (mark/pitch). The solid curve 1610 is a plotof the resonance frequency of the A1-3 mode as a function of IDTmark/pitch for an XBAR with a z-cut lithium niobate diaphragm 400 nmthick. The solid curve 1610 is read against the left vertical axis ofthe graph. The IDT conductors are aluminum 100 nm thick and the IDTpitch is 3 μm. The A1-3 mode resonance frequency depends on IDTmark/pitch. As shown in FIG. 9, varying the IDT mark/pitch from 0.15 to0.45 μm results in reduction in resonance frequency by about 10%.

The dashed curve 1620 is a plot of the absolute admittance of the A1-3mode as a function of IDT mark/pitch. The dashed curve 1620 is readagainst the right vertical axis of the graph. The relationship betweenadmittance and mark/pitch is nonlinear. Minimum admittance occurs forIDT mark/pitch from 0.235 to 0.265. The admittance increases formark/pitch values outside of this range but is still small for amark/pitch range from 0.2 to 0.3.

The resonance and anti-resonance frequencies of the primary acousticmode of an XBAR is determined by multiple factors including the type,crystallographic orientation, and thickness of the piezoelectric slaband the pitch and mark of the IDT fingers. In particular, differentcombinations of mark and pitch on the same piezoelectric diaphragm canexcite the same primary acoustic mode. In this context, two acousticmodes are considered to be the same if the two acoustic modes have thesame direction of acoustic energy flow and the same resonance and/oranti-resonance frequencies.

A radio frequency or microwave signal applied across the IDT of an XBARmay also excite undesired spurious acoustics modes. The frequency andstrength of such spurious acoustic modes also depend on multiple factorsincluding the pitch and mark of the IDT fingers. However, two or moremark/pitch combinations that excite the same primary acoustic mode donot necessarily excite the same spurious modes. When the pitch and markwithin an IDT is changed between two or more mark/pitch combinationsthat produce the same primary acoustic mode but different spuriousmodes, the different spurious modes will not add constructively over thearea of the IDT.

The XBAR primary acoustic mode is mostly bulk in nature, which canresult in weak frequency dependence on mark and pitch. Thus, chirping(or variance) of mark, or mark and pitch, in the IDT of the XBAR canpotentially suppress undesirable spurious modes that depend upon markand/or pitch, such as metal and propagating modes, with only slightbroadening of the primary mode resonance.

FIG. 17 is an expanded schematic plan view of a unit cell 1700 for anIDT where the pitch and mark vary along the length of the IDT. The scaleof the plan view has been stretched or expanded horizontally for ease ofpresentation of the various dimensions. In the unit cell 1700, P₁ is afirst pitch value, P₂ is a second pitch value, M₁ is a first mark valueand M₂ is a second mark value. The differences between P₁ and P₂ andbetween M₁ and M₂ are exaggerated for ease of visualization. An IDThaving a pitch P₁ and a mark M₁ would excite a primary acoustic modewith particular resonance and antiresonance frequencies. An IDT having apitch P₂ and a mark M₂ would excite the same primary acoustic mode,which is to say a primary acoustic mode with the same resonance and/orantiresonance frequencies.

The unit cell 1700 includes a first set of IDT fingers 1712, 1714, 1716extending from an upper busbar 1710, and a second set of IDT fingers1722, 1724 extending from a lower busbar 1720. In this patent,directional terms such as upper, lower, left, right, vertical,horizontal, etc. refer to direction or position within the drawing beingdiscussed and do not imply any physical position or orientation. Theunit cell 1700 is intended to be cascaded in the horizontal direction(as will be described in conjunction with FIG. 18) to form an IDT. Theunit cell 1700 only includes portions of the end fingers 1712 and 1716.Other portions of those fingers exist within adjacent unit cells (notshown in FIG. 17).

The unit cell 1700 is divided into a first pitch/mark zone 1730 and asecond pitch/mark zone 1740. Within the first pitch/mark zone 1730, thepitch between adjacent fingers is P₁ and the mark of the fingers is M₁.Within the second pitch/mark zone 1740, the pitch between adjacentfingers is P₂ and the mark of the fingers is M₂. The mark/pitchcombination of the IDT changes between M₁/P₁ and M₂/P₂ every twofingers. To this end, each finger 1712, 1714, 1716 of first set of IDTfingers extending from the upper busbar 1710 has a uniform width of(M₁+M₂)/2, including portions of fingers 1712 and 1716 within adjacentunit cells. Note that the pitch is not measured to the respectivecenters of the first set of IDT fingers, but to a dashed line thatdivides each finger in a ratio of M₁/M₂. For example, finger 1714 isdivided such that the portion of the finger extending left into the1^(st) pitch/mark zone 1730 has a width of M₁/2, and the portion of thisfinger extending right into the 2^(nd) pitch/mark zone 1740 has a widthof M₂/2. The second set of IDT fingers 1722, 1724 extending from thelower busbar 1720 has a center-to-center distance between adjacentfingers equal to P₁+P₂. The mark of the second set of IDT fingers 1722,1724 alternates between M₁ (e.g. finger 1722) and M₂ (e.g. finger 1724).The net effect is that the unit cell 1700 has two periods of pitch P₁and mark M₁ followed by two periods of pitch P₂ and mark M₂.

The unit cell 1700 may be cascaded to provide an IDT with any desiredlength with a corresponding number of fingers. FIG. 18 is a schematicplan view of an IDT 1800 composed of eight copies 1700A to 1700H of theunit cell 1700 juxtaposed along the length of the IDT 1800 resulting ina total of 33 fingers. The IDT 1800 is shown reasonably to scale(approximately 1600:1), with the exception that the differences betweenP₁ and P₂ and between M₁ and M₂ are still exaggerated for ease ofvisualization. The IDT 1800 has eight first pitch/mark zones 1830A to1830H in which the pitch and mark are P₁ and M₁, respectively. The eightfirst pitch/mark zones 1830A to 1830H are interleaved with eight secondpitch/mark zones 1840A to 1840H, in which the pitch and mark are P₂ andM₂, respectively. A pattern of two periods of pitch P₁ and mark M₁alternating with two periods of pitch P₂ and mark M₂ is continued alongthe length of the IDT. Using eight copies of the unit cell 1700 isexemplary and an IDT may use more or fewer than 8 unit cells and havemore or fewer than 33 fingers.

FIG. 19 is a plan view of an exemplary multi-mark IDT 1900. A“multi-mark IDT” is an IDT where the mark of the IDT fingers variesalong the length of the IDT. At any given point along the length, themark may not vary across the aperture of the IDT. Further, the pitch,can be constant over the entire IDT. The multi-mark IDT 1900 may be aportion of an XBAR such as the XBAR 100 of FIG. 1.

The multi-mark IDT 1900 includes a first busbar 1932, and a secondbusbar 1934, and a plurality of interleaved fingers such as finger 1936.The interleaved fingers extend alternately from the first and secondbusbars 1932, 1934. The multi-mark IDT 1900 is divided into threesections, identified as Section A, Section B, and Section C, along thelength L of the IDT. Each of Sections A, B, and C includes 20 fingers,for a total of 60 fingers in the multi-mark IDT 1900. The use of threesections and 60 fingers is exemplary. An IDT may have more than or fewerthan 60 total fingers. An IDT may be divided along its length into twoor more sections, each of which includes a plurality of adjacentfingers. The total number of fingers may be divided essentially equallybetween the two or more sections. In this context, “essentially” means“as close as possible.” For example, an IDT with 100 fingers dividedinto three sections with 33, 34, and 33 fingers is considered to bedivided essentially equally. The total number of fingers may be dividedunequally between the two or more sections.

In this example, the fingers in Section B have mark m, which is thenominal mark of the IDT. The finger of Section A have a mark ofm(1−δ_(m)), and the fingers of Section C have a mark of m(1+δ_(m)).δ_(m) is greater than 0 and less than or equal to 0.05. δ_(m) maytypically be less than 0.01. δ_(m) may be selected during a filterdesign to achieve the most effective reduction of spurious modes. At anypoint along the length L of the IDT 1900, the mark is constant acrossthe aperture A. The pitch of the IDT fingers is constant and the same inall sections. When an IDT is divided into two sections or more thanthree sections, the maximum mark may be m(1+δ_(m)) and the minimum markmay be m(1−δ_(m)).

In the example multi-mark IDT 1900, the mark increases monotonicallyfrom left (as seen in the figure) to right. This is not necessarily thecase in all multi-mark IDTs. The sections of a multi-mark IDT may bearranged in some other order. Further, in the multi-mark IDT 1900, thechange in mark between adjacent sections is constant. This is also notnecessarily the case in all multi-mark IDTs. The change in mark betweenadjacent sections may be the same or different.

FIG. 20 is a plan view of another multi-mark IDT 2000 with continuouslyvarying mark. The IDT 2000 includes a first busbar 2032, and secondbusbar 2034, and a plurality of interleaved fingers such as finger 2036.The interleaved fingers extend alternately from the first and secondbusbars 2032, 2034. The IDT 2000 is not divided into sections, butrather has a continuous change in mark for the fingers 2036 along itslength L. The IDT 2000 has 60 fingers, which is exemplary. An IDT mayhave more than or fewer than 60 total fingers. The multi-mark IDT 2000may be a portion of an XBAR such as the XBAR 100 of FIG. 1.

As shown in FIG. 20, the mark at the left edge of the IDT 500 ism(1−δ_(m)), and the mark at the right edge of the IDT 500 is m(1+δ_(m)).The mark varies continuously between these two extremes. The variationin mark may typically, but not necessarily, be a linear function ofposition along the length L of the IDT. δ_(m) is greater than 0, lessthan or equal to 0.05, and typically less than 0.01. δ_(m) may beselected during a filter design to achieve the most effective reductionof spurious modes. At any point along the length of the IDT 2000, themark is constant across the aperture A. The pitch of the IDT fingers isconstant over the entire IDT.

Slight variations in the mark of the IDT in an XBAR can result indisruption or destructive interference of spurious modes with negligibleeffect on the shear primary mode.

Multi-mark IDTs may be divided into two sections or more than threesections, or may be continuous. The number of sections may not be thesame for all resonators in a filter, and a filter may include bothsectioned and continuous multi-mark IDTs. The value of δ_(m) may bedifferent for some or all of the resonators. A filter may contain acombination of resonators with uniform mark and multi-mark resonators.

In other examples, the pitch of the IDT can vary continuously, similarto the continuous variation of mark shown in FIG. 20. The pitch may varyas the mark varies, or may vary at a different rate. The pitch and markmay both vary continuously. The pitch and/or the mark may vary betweenmultiple maxima and minima along the length of the IDT. The mark mayvary by section while the pitch varies continuously, or pitch may varyby section while the mark varies continuously. The mark may increase inone direction along the length of the IDT (either continuously or bysection), while the pitch decreases in the same direction (eithercontinuously or by section). Variation of mark and pitch can beoptimized with respect to one another, and variation of mark and pitchcan be different from one resonator to another, such that greatestsuppression of spurious modes is achieved for best performance of thefilter.

FIG. 21 is a graphical illustration of Euler angles 2100. Euler anglesare a system, introduced by Swiss mathematician Leonhard Euler, todefine the orientation of a body with respect to a fixed coordinatesystem. The orientation is defined by three successive rotations aboutangles α, β, and γ.

As applied to acoustic wave devices, xyz is a three-dimensionalcoordinate system aligned with the crystalline axes of the piezoelectricmaterial. XYZ is a three-dimensional coordinate system aligned with theacoustic wave device, where Z is normal to the surface of thepiezoelectric material. XY is the plane of the surface of thepiezoelectric material. X is the direction of the electric field andacoustic wave propagation for SAW and most plate-mode devices, and Y istypically parallel to the fingers on an IDT. In XBAR devices, X is thedirection of the electric field, but acoustic wave propagation isdominantly along the Z direction. All of the XBAR devices described inapplication Ser. Nos. 16/230,443 and 16/381,141 use piezoelectric plateswith the z axis normal to the plate surface and the y axis orthogonal tothe IDT fingers. Such piezoelectric plates have Euler angles of 0, 0,90°.

For historical reasons, lithium niobate plates having Euler angles [0°,β, 0°] are commonly referred to as “Y-cut”, where the “cut angle” is theangle between the y axis and the normal to the plate. The “cut angle” isequal to β+90°. For example, a plate with Euler angles [0°, 30°, 0° ] iscommonly referred to as “120° rotated Y-cut”.

FIG. 22 is a graph 2200 of two piezoelectric coefficients e15 and e16for lithium niobate plates having Euler angles [0°, β, 0° ]. The solidline 2210 is a plot of piezoelectric coefficient e15 relating electricfield along the x axis to shear stress or torque εxz axis as a functionof β. This shear stress excites the shear primary acoustic mode shown inFIG. 3. The dashed line 2220 is a plot of piezoelectric coefficient e16relating electric field along the x axis to shear stress or torque εxyas a function of β. This shear stress excites horizontal shear modes(e.g. the SH0 plate mode) with atomic displacements normal to the planeof FIG. 3, which are undesired parasitic modes in an XBAR. Note thatthese two curves are identical and shifted by 90°, (as y-axis shiftedfrom x-axis).

Inspection of FIG. 22 shows that the first piezoelectric stresscoefficient is highest for Euler angle β about 30°. The firstpiezoelectric stress coefficient is higher than about 3.8 (the highestpiezoelectric stress coefficient for an unrotated Z-cut lithium niobate)for 0°≤β≤60°. The second piezoelectric stress coefficient is zero forEuler angle β about 30°, where the first piezoelectric stresscoefficient is maximum. In this context “about 30°” means “within areasonable manufacturing tolerance of 30°”. The second piezoelectricstress coefficient is less than about 10% of the first piezoelectricstress coefficient for 26°<(3<34°.

Shear wave velocity and diaphragm thickness are both temperaturedependent, with the temperature coefficient of shear wave velocity (TCV)being the dominant factor in the temperature dependence of resonancefrequency.

The difference between the resonance and anti-resonance frequencies ofan XBAR is determined, in part, by the electro-mechanical couplingbetween the electric field and the primary shear wave. This couplingdepends on piezoelectric coupling coefficient e₁₅. e₁₅ is an element ofa 3×6 matrix of piezoelectric coupling coefficients that describe thephysical response of a piezoelectric material to an applied electricfield. A larger value of e₁₅ results in more efficient coupling to theprimary shear acoustic mode, which results in wider spacing between theresonance and anti-resonance frequencies of an XBAR.

FIG. 23 is a chart 2300 of piezoelectric coupling coefficient e₁₅ andtemperature coefficient of velocity TCV as functions of Euler angle βfor lithium niobate with Euler angles (0°, β, 0°). Specifically, thesolid curve 2310 is a plot of TCV versus β. The solid curve 2310 is readagainst the left-hand axis. TCV is expressed in part per million perdegree Celsius (ppm/° C.). The dashed curve 2320 is a plot of e₁₅ versusβ. The dashed curve 2320 is read against the right-hand axis.

Lithium niobate crystal orientations previously used for XBARs includeZ-cut and rotated Y-cut. Z-cut has Euler angles=(0°, 0°, 90°). RotatedY-cut has Euler angles=(0°, β, 0°), with β between 30 and 38 degrees.Z-cut lithium niobate has a TCV of about −102 ppm/° C. and e₁₅ of about3.7. Rotated Y-cut lithium niobate has e₁₅ about 4.4 and TCV betweenabout −86 ppm/° C. and −92 ppm/° C.

Inspection of FIG. 23 shows that rotated Y-cut lithium niobate with (3about 67 degrees (broken line 432) has a value of e₁₅ of about 3.7(broken line 434) which is equivalent to the e₁₅ for Z-cut lithiumniobate. Rotated Y-cut lithium niobate with β about 67 degrees has a TCVof about −73 ppm/° C., which is 30% smaller (in magnitude) than the TCVof Z-cut lithium niobate. Filters comprised of XBARs using lithiumniobate piezoelectric plates with β substantially equal to 67° may haveperformance comparable to filters using Z-cut lithium niobate withsignificantly less frequency dependence on temperature. In this andsimilar contexts, “substantially equal” means equal with definedmanufacturing tolerances. The range from β=38° to β=67° offers acontinuous trade-off between piezoelectric coupling and TCV. Forexample, a rotated Y-cut lithium niobate plate with β=60° offers 5%higher piezoelectric coupling than a plate with β=67° with only a smallincrease in the magnitude of TCV.

The bandwidth and other requirements of a particular filter may dictatea minimum value for e₁₅. The Euler angles (0°, β, 0°) of thepiezoelectric plate may be selected with β set to the highest value inthe range from 40° to 67° that provides the required minimum value ofe₁₅, while minimizing, to the extent possible, the TCF of the filter.

FIG. 24 is a chart 2400 of the electromechanical coupling coefficient ofrepresentative XBAR devices using piezoelectric plates with Euler angles0, β, 90°, where β is in the range from −15° to +5°. The chart in FIG.24 is based on simulation of XBAR devices using finite element methods.The line 2410 is a plot of electromechanical coupling coefficient as afunction of β. Inspection of the chart 2400 shows that theelectromechanical coupling coefficient is greater than 0.26 for βgreater than or equal to −11° and less than or equal to −5°, as comparedto a value of about 0.243 for β=0.

The Q-factor of an acoustic resonator is commonly defined as the peakenergy stored during a cycle of the applied RF signal divided by thetotal energy dissipated or lost during the cycle. The Q-factor of anXBAR is a complex function of numerous parameters including the length,or number of fingers, in the IDT of the XBAR.

Possible loss mechanisms in an acoustic resonator include resistivelosses in the IDT and other conductors; viscous or acoustic losses inthe piezoelectric plate, IDT fingers, and other materials; and leakageof acoustic energy out of the resonator structure. The peak energystored in a resonator is proportional the capacitance of the resonator.In an XBAR resonator, the capacitance is proportional to the number ofIDT fingers. Resistive losses and viscose losses are also proportionalto the number of IDT fingers. Acoustic energy that leaks from theresonator in the transverse direction (i.e. the direction parallel tothe IDT fingers) is proportional to the length of the resonator and thusalso proportional to the number of IDT fingers. In contrast, energy lostfrom the ends of the IDT in the longitudinal direction (i.e. thedirection normal to the IDT fingers) is roughly constant, independent ofthe number of IDT fingers. As the number of IDT fingers and the peakenergy stored in an XBAR is reduced, the acoustic energy lost in thelongitudinal direction becomes an ever-increasing fraction of the peakenergy stored.

FIG. 25 is a plan view of an exemplary conductor pattern 2500 thatreduces the acoustic energy leakage in the longitudinal direction at theends of an XBAR. The conductor pattern 2500 includes an IDT 2530 andfour reflector elements 2562, 2564, 2566, 2568. The IDT 2530 includes afirst busbar 2532, a second busbar 2534, and a plurality of ninterleaved IDT fingers extending alternately from the first and secondbusbars. In this example, n, the number of IDT fingers, is equal to 24.In other XBARs, n may be in a range from 20 to 100 or more IDT fingers.IDT finger 2536 is the 1^(st) finger and IDT finger 2538 is the n'thfinger. Numbering the IDT fingers from left to right (as shown in FIG.25) is arbitrary and the designations of the 1^(st) and n'th fingerscould be reversed.

As shown in FIG. 25, the odd numbered IDT fingers extend from the firstbusbar 432 and the even numbered IDT fingers extend from the secondbusbar 2534. The IDT 2530 has an even number of IDT fingers such thatthe 1^(st) and n'th IDT fingers 2536, 2538 extend from differentbusbars. In some cases, an IDT may have an odd number of IDT fingerssuch that the 1^(st) and n'th IDT fingers and all of the reflectorelements extend from the same busbar.

A total of four reflector elements are provided outside of periphery ofthe IDT 2530. A first reflector element 2562 is proximate and parallelto 1st IDT finger 2536 at the left end of the IDT 2530. A secondreflector element 2566 is proximate and parallel to n'th IDT finger 2538at the right end of the IDT 2530. An optional third reflector element2564 is parallel to the first reflector element 2562. An optional fourthreflector element 2568 is parallel to the second reflector element 2566.

First and third reflector elements 2562, 2564 extend from the firstbusbar 2532 and thus are at the same electrical potential as the 1st IDTfinger 2536. Similarly, second and fourth reflector elements 2566 and2568 extend from the second busbar 2530 and thus are at the sameelectrical potential as the n'th IDT finger 2538.

The reflector elements 2562, 2564, 2566, 2568 are configured to confineacoustic energy to the area of the IDT 2530 and thus reduce acousticenergy losses in the longitudinal direction. To this end, the pitch prbetween adjacent reflector elements and between reflector elements 2562and 2566 and the adjacent first and n'th IDT fingers, respectively, istypically greater than the pitch p of the IDT fingers. The width or markmr of the reflector elements 2562, 2564, 2566, 2568 is not necessarilyequal to the mark m of the IDT fingers. As will be describedsubsequently, the mark mr of the reflector elements may be selected tooptimize Q-factor at a specific frequency or range of frequencies.

In other embodiments, reflector elements (e.g., four reflector elementsare provided outside of periphery of the IDT. First and third reflectorelements are proximate and parallel to 1st IDT finger at the left end ofthe IDT that are connected to each other but are not connected to eitherbusbar. First and third reflector elements are capacitively coupled to1st IDT finger and thus are at substantially the same electricalpotential as the 1st IDT finger. The reflector elements are consideredto be at substantially the same potential if, when an RF signal isapplied between the busbars, the potential between the reflectorelements and the 1^(st) IDT finger is small compared to the potentialbetween adjacent IDT fingers. Similarly, second and fourth reflectorelements are proximate and parallel to n'th IDT finger at the right endof the IDT. Second and fourth reflector elements are connected to eachother and not connected to either busbar. Second and fourth reflectorelements are capacitively coupled to the n'th IDT finger and thus are atnearly the same electrical potential as the n'th IDT finger.

FIG. 26A is a schematic cross-sectional view of an XBAR device 2600Awith an etch-stop layer and back-side etched cavities. The XBAR device2600A includes two XBARs, each of which is similar to the XBAR 100 ofFIG. 1. A back surface 2614 of a piezoelectric plate 2610 is attached toa substrate 2620. An electrode pattern is formed on a front surface 2612of the piezoelectric plate 2610. The electrode pattern includesinterleaved fingers 2630 of respective IDTs for the two XBARs. The IDTfingers 2630 are disposed over respective cavities 2640A formed in thesubstrate 2620. The materials of the piezoelectric plate, substrate, andelectrode pattern are as previously described.

The primary difference between the XBAR device 2600A and the XBAR 100 ofFIG. 1 is the presence of an etch-stop layer 2650 sandwiched between thepiezoelectric plate 2610 and the substrate 2620. The term “sandwiched”means the etch-stop layer 2650 is both disposed between and physicallyconnected to a surface of the substrate 2620 and the back surface 2614of the piezoelectric plate 2610. In some embodiments, as will bedescribed subsequently, layers of additional materials may be disposedbetween the etch-stop layer 2650 and the surface of the substrate 2620and/or between the etch-stop layer 2650 and the back surface 2614 of thepiezoelectric plate 2610. In XBAR device 2600A, the piezoelectric plate2610 is not bonded directly to the substrate 2620 but is attached to thesubstrate 2620 via the etch-stop layer 2650.

The cavities 2640A are formed by using an etch process to removematerial from the substrate. The etch process may be a “wet” processusing a liquid etchant, or a “dry” process such as reactive ion etchingor sputter etching that use a gaseous etchant. As represented by thedashed arrow 2660A, the etch process proceeds from the back surface ofthe substrate and progressively removes material from the substrateuntil the cavities 2640A are formed. In the absence of the etch-stoplayer 2650, at least a portion of the back surface 2614 of thepiezoelectric plate 2610 would be exposed to the etch process 2660. Theperformance of the XBAR 2600A is sensitive to the thickness of thepiezoelectric and, to at least some extent, to the smoothness of theback surface 2614. Any erosion of the back surface 2614 by the etchprocess 2660 may have a deleterious effect on the performance of theXBAR 2600A.

The etch-stop layer 2650 protects the back surface 2614 from the etchprocess. To this end, the etch-stop layer 2650 is impervious to the etchprocess represented by the dashed arrow 2660A. The word “impervious” hasseveral definitions including “not affected by” and “not allowing fluidto pass though”. Both of these definitions apply to the etch-stop layer2650. The etch-stop layer is not materially affected by the etch processand does not allow the liquid or gaseous etchant used in the etchprocess to penetrate to the piezoelectric layer 2610. The etch-stoplayer need not be inert with respect to the etchant but must be highlyresistant to the etchant such that a substantial portion of the etchstop layer remains after completion of the cavity etch. The remainingetch stop layer 2650 is not removed after the cavities 2640A are formedand becomes a portion of the diaphragms of the XBAR devices.

The etch-stop layer 2650 is formed from an etch-stop material. Theetch-stop material must be a dielectric with very low conductivity andlow acoustic loss. The etch-stop material must have high adhesion to thesurface(s) on which it is formed. Further, the etch-stop material mustbe compatible with attaching the piezoelectric plate to the substratewith a wafer bonding process. Most importantly, the etch-stop materialmust be impervious, as previously defined, to the processes andchemicals used to etch the substrate material. Suitable etch-stopmaterials may include oxides such as aluminum oxide and silicon dioxide,sapphire, nitrides including silicon nitride, aluminum nitride, andboron nitride, silicon carbide, and diamond.

When the etch-stop material is a high thermal conductivity dielectric,such as aluminum nitride, boron nitride, or diamond, the etch-stop layerwill help conduct heat away from the diaphragm of the XBAR.

As described in U.S. Pat. No. 10,491,192, a dielectric layer 2670 may beselectively formed on the front side of the piezoelectric plate 2610over the IDTs 2630 of some XBARs. For example, a frequency settingdielectric layer may be formed over the IDTs of shunt resonators tolower their resonant frequencies with respect to the resonantfrequencies of series resonators in a filter. The electromechanicalcoupling efficiency of an XBAR may be reduced and spurious modes may beenhanced if the total thickness of dielectric layers on the front andback surfaces of the piezoelectric plate exceeds about 35% of thepiezoelectric plate thickness. Further, filters designed for broadcommunications bands such as band n77 and band n79 may require afrequency setting layer with a thickness of 20% to 30% of thepiezoelectric plate thickness. To allow flexibility in selection of thefrequency setting layer thickness, the thickness t_(es) of the etch-stoplayer 2650 may be less than or equal to 10% of the piezoelectric platethickness, and preferably about 4% to 6% of the piezoelectric platethickness. When a frequency setting dielectric layer is not used, thethickness t_(es) may be less than about 20% of the piezoelectric platethickness.

FIG. 26B is a schematic cross-sectional view of an XBAR device 2600Bwith an etch-stop layer and front-side etched cavities. The XBAR device2600B includes two XBARs, each of which is similar to the XBAR 100 ofFIG. 1. A back surface of a piezoelectric plate 2610 is attached to asubstrate 2620. An etch-stop layer 2650 is sandwiched between thepiezoelectric plate 2610 and the substrate 2620. An electrode pattern isformed on a front surface of the piezoelectric plate 2610. The electrodepattern includes interleaved fingers 2630 of respective IDTs for the twoXBARs. The IDT fingers 2630 are disposed over respective cavities 2640Bformed in the substrate 2620. The materials and characteristics of thepiezoelectric plate 2610, substrate 2620, etch-stop layer 2650, andelectrode pattern 2630 are as previously described.

The primary difference between the XBAR device 2600B and the XBAR device2600A of FIG. 26A is the etch process used to form the cavities 2640B.The cavities 2640B are formed with an etch process, represented by thedashed arrow 2660B, using an etchant introduced though openings 2642 inthe piezoelectric plate 2610 and the underlying etch-stop layer 2650.

FIG. 27 shows a schematic cross-sectional view and two detailedcross-sectional views of a filter 2700 using XBARs. A piezoelectricplate 2710 is attached to a substrate 2720. An optional dielectric layer2725 may be between the piezoelectric plate 2710 and the substrate 2720.A portion of the piezoelectric plate 2710 forms a diaphragm 2715spanning a cavity 2740 in the substrate. As shown, the cavity 2740 doesnot fully penetrate the substrate 2720. Alternatively, the cavity 2740may penetrate the substrate as shown in FIG. 1. Fingers of an IDT aredisposed on the diaphragm 2715. Two conductors 2750 and 2755 are formedon the surface of the piezoelectric plate 2710 at a location removedfrom the cavity 2740. The two conductors 2750, 2755 may be signalconductors interconnecting XBARs and/or other components of the filter2700. The conductors 2750 and 2755 may be a signal conductor and aground conductor. While FIG. 27 only shows a single XBAR and twoconductors, a filter may include multiple XBARs and more than two signaland ground conductors.

A preferred material for the substrate 2720 is silicon. Silicon wafersare readily available and inexpensive. Further, processes and equipmentfor handling silicon wafers are well developed. However, silicon is asemiconductor material. Silicon wafers may be doped, or loaded withimpurities, to have a desired bulk resistivity. Undoped, or intrinsic,silicon wafers can form a conductive inversion layer along the boundarybetween the silicon and some other material, such as along the boundaryof the silicon wafer 2720 and the dielectric layer 2725 of the filterdevice 2700. If the dielectric layer 2725 is not present, the inversionlayer may form along the boundary between the silicon wafer 2720 and thepiezoelectric plate 2710.

As shown in Detail A of FIG. 27, conductors 2750 and 2755 arecapacitively coupled to the substrate 2720 through the piezoelectricplate 2710 and the dielectric layer 2725, if present. If the substrate2720 is conductive, or if a conductive inversion layer is formed in thesubstrate 2720, the conductors 2750, 2755 will be effectively connected,at RF frequencies, by a parasitic resistance 2760. Power dissipated inthe resistance 2760 will contribute to the insertion loss of the filter2700.

Detail B of FIG. 27, shows a cross-sectional view of a portion of afilter formed on a substrate 2720 including a high resistivity siliconwafer 2722 and a trap-rich region 2724. The trap rich region 2724 may bea region within the high resistivity silicon wafer 2722 or a layerformed on a surface of the high resistivity silicon wafer 2722. Ineither case, the trap-rich region is immediately adjacent the dielectriclayer 2725 or the piezoelectric plate 2710 if the dielectric layer 2725is not present. The trap-rich region 2722 has an abundance of traps thatcapture free carriers and reduce carrier lifetime to an extent that theconductivity of the trap-rich region approaches zero.

A trap-rich region may be formed within a silicon substrate byirradiating the surface of the substrate with neutrons, protons, orvarious ions (silicon, argon, nitrogen, neon, oxygen, etc.) to createdefects in the crystalline structure of the substrate. Alternatively, atrap-rich region may be formed within a silicon substrate by introducingdeep trap impurities such as gold, copper, or other metal ions. Suchimpurities may be introduced by ion implantation, diffusion, or someother method. The trap-rich region may be formed by a combination ofthese techniques. When the dielectric layer 2725 is included in thefilter 2700, the trap-rich region 2724 may be formed before thedielectric layer is formed on the substrate 2720. Alternatively, thetrap-rich region 2724 may be form by ion implantation through thedielectric layer 2725.

A trap-region layer may be formed on a silicon substrate by depositing alayer of trap-rich material such as amorphous silicon or polysilicon(polycrystalline silicon). When the trap-rich region is polysilicon, theaverage grain size of the polysilicon should be substantially smallerthan the minimum spacing between electrodes 2750, 2755. The thickness ofthe trap rich region formed on or within a high resistivity siliconsubstrate should be greater than the thickness of an inversion layerthat may form in the absence of the trap-rich layer.

FIG. 28 is a plan view of an XBAR 2800 with periodic etched holes. TheXBAR 2800 includes a piezoelectric plate 2810 having parallel front andback surfaces 2812, 2814, respectively. The piezoelectric plate is athin single-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent.

The back surface 2814 of the piezoelectric plate is attached to surfaceof a substrate 2820. A portion of the piezoelectric plate forms adiaphragm spanning a cavity 2840 in the substrate 2820. As shown in FIG.28, the cavity 2840 extends completely through the substrate 2820. Thecavity may only extend part way through the substrate, as shown in FIG.2A and FIG. 2B.

An IDT 2830 is formed on the surface of the piezoelectric plate 2810.The IDT 2830 includes a first busbar 2832 and a second busbar 2834. Afirst set of parallel fingers, such as finger 2836 extends from thefirst busbar 2832. A second set of parallel fingers extends from thesecond busbar 2834. The first and second sets of fingers are paralleland interleaved. At least the interleaved fingers of the IDT aredisposed on the diaphragm. A periodic array of holes 2880 are formed inthe diaphragm. As shown in FIG. 28, the periodic array includes one holeat the end of each IDT finger. Specifically, a hole is disposed betweenthe end of each of the first set of fingers and the second busbar and ahole is disposed between the end of each of the second set of fingersand the first busbar. Other periodic arrangements of the holes, such asat the ends of alternate IDT fingers may be used.

The periodic array of holes 2880 has two effects on the performance ofthe XBAR 2800. First, the holes scatter, and thus inhibit resonance of,spurious acoustic waves traveling parallel to the IDT fingers. Suchspurious acoustic waves can introduce ripple in the input/outputtransfer function of XBAR filters. Second, the array of holes 2880appears to increase the Q-factor of XBAR devices, possibly by helping toconfine the primary shear acoustic mode to the aperture of the XBAR.

As shown in FIG. 28, the holes 2880 are right circular cylinders with adiameter approximately equal to the width of the IDT fingers. The sizeand shape of the holes in FIG. 28 is exemplary. The holes may be largeror smaller than the width of the IDT fingers and may have across-sectional shape other than circular. For example, thecross-sectional shape of the holes may be oval, square, rectangular, orsome other shape. The holes need not necessarily pass through thepiezoelectric plate. The holes may be blind holes that only extend partway though the thickness of the piezoelectric plate. The size and depthof the holes must be sufficient to create a domain with significantlyreduced acoustic impedance. An additional benefit of holes at the endsof the IDT fingers is reduction of parasitic capacitance between the IDTfinger tips and the adjacent busbar.

To produce improved XBAR resonators and filters that efficiently conductheat from the IDT or busbars to the substrate, predetermined areas ofthe bonding oxide layer (e.g., BOX) and/or piezoelectric layer can beremoved from selected locations of the surface of the substrate of thedevice to provide a predetermined amount in reduction of thermalresistance between the conductor pattern and the substrate. Thepredetermined areas removed from the selected locations may be describedas excess BOX and piezoelectric material that is removed because theirremoval does not affect or change the filtering performance (e.g.,frequency range passed) by the filter. In some cases, the removal maychange the performance by less than 5 percent of the frequency rangeand/or wave pass amplitude. In some cases, it is by less than 10percent. It may be by less than 3 percent.

In some cases, to produce improved XBAR resonators and filters withexcess BOX and piezoelectric material removed, the portions or areas ofthe BOX and piezoelectric material of a plate or layer that extend acertain distance past the cavity perimeter of the cavities of filter maybe removed. This removing may include removing the BOX and piezoelectricmaterial: a) that extends in the length direction past the perimeter ofthe cavity by between 2 and 25 percent more the length of the cavity;and b) that extends in the width direction past the perimeter of thecavity by between 2 and 25 percent more the width of the cavity. Thisremoving may include removing the excess BOX and piezoelectric materialto provide a predetermined amount in reduction of thermal resistancebetween the conductor pattern and the substrate. This removing mayinclude removing the excess BOX and piezoelectric material fromlocations immediately adjacent to (e.g., touching) contact layers and/orunder contact bumps. It may include removing the BOX and piezoelectricmaterial from outside of the XBAR resonators or diaphragms of an RFfilter, such as from locations beside a cavity over which the resonatoror diaphragm spans (e.g., is suspended or extends).

FIG. 29A is a schematic cross-sectional view of XBAR device 2900 havingpredetermined areas of the bonding oxide layer 2922 (e.g., BOX layer)and piezoelectric layer 2910 removed from selected locations over thesurface of the substrate of the device to provide a predetermined amountin reduction of thermal resistance between the IDT pattern 2936 and thesubstrate 2920. FIG. 29A may be a view of the filter device at thesection A-A of FIG. 1, section B-B of FIG. 1, and/or of plane A-A ofFIG. 2A. FIG. 29A shows filter device 2900 comprising substrate 2920having cavity 2940. BOX layer 2922 is formed on the substrate and spansover the cavity 2940. Piezoelectric plate 2910 is bonded to the bondinglayer 2922 and spans the cavity 2940. In some cases, bonding layer 2922does not exist over the cavity 2940 and is only between where the plateis attached to the substrate.

An interdigital transducer (IDT) 2930 formed on a front surface of thepiezoelectric plate 2910 has busbars 2932 and 2934; and interleavedfingers 2936. Each of the busbars is attached to a set of fingers thatform interleaved fingers 2936. Fingers 2936 may span or be over cavity2940. In some cases, part of the busbars of the IDT are also over thecavity. In other cases, all of the busbars are over the substrate 2920but not over the cavity. At least portions of the busbars are over thesubstrate (e.g., not over the cavity) to better conduct heat generatedin the IDT to the substrate.

Device 2900 has width WC of cavity 2940; width WP of layers 2922 and2910; width WIDT of IDT 2930. It also has lengths of the cavity, layersand IDT that are related to the widths, such as noted in FIG. 1. Thewidths and corresponding length may define a perimeter of the cavity,layers (e.g., diaphragm) and IDT.

Second metal layers 2970 and 2971 of M2 material are attached to the topof the substrate 2920; to the side surfaces of bonding layer 2922; tothe side surfaces and part of the top surface of the piezoelectric layer2910; and to the side surfaces and part of the top surface of the IDT2936, such as to the top of the busbars and not to the top of thefingers 2936. In some cases, the second metal layers 2970 and 2971 are asingle metal layer extending around a perimeter of and form an island ofthe bonding layer 2922, plate 2910 and IDT 2930.

The materials that can be used for the substrate, bonding layer,piezoelectric plate/layer, IDT, fingers, busbars, conductor pattern ofFIGS. 5A and 5B may be the same as those described for FIG. 1. Thematerial of layer M1 and layer M2 of FIGS. 5A-5B may be the same asthose described for FIG. 1. The bonding layer 2922 may be BOX such assilicon dioxide, Al₂O₃, silicon nitride, silicon carbide, SiOC, aluminumnitride, a metal oxide, another oxide or another proper bondingmaterial. It can be multiple layers of one or more of such materials.

The heat generated in or by the diaphragm 2910 during filtering may beconducted through the fingers 2936 to busbars 2932 and 2934 of the IDT2930; and then to the substrate 2920. However, the busbars and otherparts of the IDT are separated from the substrate by the piezoelectriclayer and a layer of bonding oxide (BOX).

Thus, an improved XBAR resonator 2900 that efficiently conducts heatfrom the fingers 2936 and busbars 2932 and 2934 to the substrate 2920 isformed by removing predetermined areas (e.g., excess amounts) WR1 andWR2 of the bonding layer 2922 and piezoelectric layer 2910 from selectedlocations of the surface of the substrate of the device 2900 to providea predetermined amount in reduction of thermal resistance between theconductor pattern and the substrate. Removing may be done by an islandetch concept that removes layers 2922 and 2910 around a perimeter of theresonator to leave an island of the layers 2922, 2910 and IDT 2930 overcavity 2940. In this case, areas WR1 and WR2 are a single area extendingaround a perimeter of and form an island of the bonding layer 2922,plate 2910 and IDT 2930.

It is considered that multiple devices 2900 may exist as islands onsubstrate 2920 with areas WR1 and WR2 separating each island. Here,layer 2970 may extend partially between each island. In other cases, itmay extend completely between all of the islands.

Removing the predetermined areas WR1 and WR2 causes the predeterminedamount of reduction in ‘contact thermal resistance’. The predeterminedamount in reduction of thermal resistance between the conductor patternand the substrate may be a reduction of 2×, 3×, 5× or 10× the thermalresistance. In some cases, it is a 3× reduction. In other cases, it is a10× reduction. The area sizes of areas WR1 and WR2 can be selected orpredetermined based on the predetermined amount of reduction desired.WR1 and WR2 may range from 1 um to 200 um, with maximum extentdetermined by resonator to resonator or resonator to bump offsets. WR1and WR2 need not be identical to each other, but are not excluded frombeing identical.

The selected locations are predetermined areas WR1 and WR2 where removalof bonding layer 2922 and piezoelectric layer 2910 take place. Forexample, the bonding layer 2922 and piezoelectric layer 2910 span thecavity and have excess portions that extend a certain length past theperimeter of the cavity. The excess portions may extend a certain lengthand width distance (WR1 and WR2) past the length and width (WC)perimeter of the cavity. The excess portions may be a perimeter of thebonding layer and piezoelectric layer that extend in the length andwidth past the perimeter of the cavity by: a) more than 5, 10 or 20percent; or b) between 2 and 25 percent of the length distance and widthdistance of the cavity. The removal regions WR1 and WR2 correspond tosome or all of the area covered by metal routing between resonators orbetween a resonators and bumps.

The IDT and second metal conductors may be metal or another properconductive material. The substrate may be silicon or another propersubstrate semiconductor material. The bonding layer may be BOX such assilicon dioxide or another proper bonding material.

The piezoelectric layer 2910 may be etched away from over layer 2910 atareas WR1 and WR2 using a selective etching technique or chemistry thatremoves layer 2910 but does not remove any of the layer 2922. Here,layer 2922 may be an etch stop layer for etching layer 2910.

The bonding layer 2922 may be etched away from over substrate 2920 atareas WR1 and WR2 using a selective etching technique or chemistry thatremoves layer 2922 but does not remove any or a functionally relevantthickness of the substrate 120. Here, substrate 2920 may be an etch stoplayer for etching layer 2922.

In some cases, both the bonding layer 2922 and piezoelectric layer 2910are etched away from over substrate 2920 at areas WR1 and WR2 using aselective etching technique or chemistry that removes both layers butdoes not remove any or a functionally relevant thickness of thesubstrate 120. Here, substrate 2920 may be an etch stop layer foretching both layers.

Removing the areas of BOX and LN layers 2910 and 2922 may not impact theelectrical isolation path of the IDT because there is no capacitancebetween M2 layers and the Si substrate layer 2920 if the BOX+LN isremoved from the areas WR1 and WR2. For example, a trap-rich layer withhigh electrical resistance formed over the surface of substrate 2920 atthe areas WR1 and WR2 will likely be sufficient to ensure the electricalisolation path. In some cases, it will be desirable to maintain highresistance with a barrier layer formed over the surface of substrate2920 at the areas WR1 and WR2, such as an oxidized Ti layer. Such alayer would be of thickness between 0 nm and 20 nm to minimize parasiticthermal resistance contributions.

FIG. 29B is a schematic cross-sectional view of XBAR device 2902 havingthree predetermined areas of the bonding layer 2922 and piezoelectriclayer 2910 removed from selected locations over the surface of thesubstrate of the device to provide a predetermined amount in reductionof thermal resistance between the IDT pattern 2936 and the substrate2920. Device 2902 may represent an alternative configuration of device2900 that includes width WP1 and another width WP2 of layers 2922 and2910; and width WR3 of a third area of the layers 2922 and 2910 that isremoved for reasons similar to removal of area WR1 and WR2. Device 2902also has bump 2972 attached to layer 2971 as shown.

Widths WP2 and WR3 may have lengths that are related to the widths, suchas noted in FIGS. 1 and 5. The widths and corresponding length maydefine a perimeter of the layers and area.

For device 2902, second metal layer 2973 of M2 material is attached tothe side surfaces of bonding layer 2922 at WP2; to the side surfaces andthe top surface of the piezoelectric layer 2910 at WP2; and to a topsurface layer 2971. It also has second metal layer 2974 of M2 materialattached to the top of the substrate 2920 at WR3; to a side surface oflayer 2973; and with bump 2972 attached to its top surface. In somecases, the second metal layers 2973 and 2974 are separate metal layersextending through trenches formed in the bonding layer 2922 and plate2910 beside the resonator or layer 2971. The BOX 2922 and LN 2910 overlength WP2 may remain in order to provide additional electricalisolation, in regions where trap-rich Si or high resistance metal filmsdo not provide sufficient electrical isolation. However, WP2 may also bezero, such that WR2 and WR3 are directly adjacent.

Thus, an improved XBAR resonator 2902 that efficiently conducts heatfrom the fingers 2936 and busbars 2932 and 2934 to the substrate 2920 isformed by removing predetermined areas (e.g., excess amounts) WR1, WR2and WR3 of the bonding layer 2922 and piezoelectric layer 2910 fromselected locations of the surface of the substrate of the device 2902 toprovide a predetermined amount in reduction of thermal resistancebetween the conductor pattern and the substrate. Removing may be done bya trench etch concept that removes layers 2922 and 2910 in trenchesbeside a perimeter of the resonator separately from or in addition tothe island of the layers 2922, 2910 and IDT 2930 described for FIG. 29A.The minimum size of WR3 is the diameter of the bump 2972, whichtypically ranges from 50 um to 100 um. WR3 may also extend as far asWR2, up to 200 um.

The selected locations and predetermined amounts for device 2902 can bethe same as for device 2900. Etching the bonding layer 2922 andpiezoelectric layer 2910 away from over substrate 2920 at the WR areascan be the same in device 2902 as for device 2900.

Description of Methods

FIG. 30 is a simplified flow chart summarizing a process 3000 forfabricating an XBAR or a filter device incorporating XBARs. The process3000 could be for fabricating a filter device including multiple XBARs,some of which may include a frequency setting dielectric or coatinglayer. The process 3000 starts at 3005 with a device substrate and athin plate of piezoelectric material disposed on a sacrificialsubstrate. The process 3000 ends at 3095 with a completed filter device.The flow chart of FIG. 30 includes only major process steps. Variousconventional process steps (e.g. surface preparation, cleaning,inspection, baking, annealing, monitoring, testing, etc.) may beperformed before, between, after, and during the steps shown in FIG. 30.

While FIG. 30 generally describes a process for fabricating a singlefilter device, multiple filter devices may be fabricated simultaneouslyon a common wafer (consisting of a piezoelectric plate bonded to asubstrate). In this case, each step of the process 3000 may be performedconcurrently on all of the filter devices on the wafer. XBARs or XBARfilter devices constructed according to this method can include any ofthe features described above. Thus, the method would include any stepsnecessary to include these features including modifications to thestructure and the dimensions of the structure, orienting of the crystalstructure of the piezoelectric plate, shaping of the electrodes andother structures related to the electrodes, etc.

The flow chart of FIG. 30 captures three variations of the process 3000for making an XBAR which differ in when and how cavities are formed inthe device substrate. The cavities may be formed at steps 3010A, 3010B,or 3010C. Only one of these steps is performed in each of the threevariations of the process 3000.

The piezoelectric plate may typically be lithium niobate, includinglithium niobite with a suitable crystal orientation. The piezoelectricplate may be some other material and/or some other cut. The devicesubstrate may preferably be silicon. The device substrate may be someother material that allows formation of deep cavities by etching orother processing.

In one variation of the process 3000, one or more cavities are formed inthe device substrate at 3010A, before the piezoelectric plate is bondedto the substrate at 3015. A separate cavity may be formed for eachresonator in a filter device. Also, the cavities can be shaped andformed such that two or more resonators can be on one diaphragm over onecavity. The one or more cavities may be formed using conventionalphotolithographic and etching techniques. Typically, the cavities formedat 3010A will not penetrate through the device substrate.

At 3015, the piezoelectric plate is bonded to the device substrate. Thepiezoelectric plate and the device substrate may be bonded by a waferbonding process. Typically, the mating surfaces of the device substrateand the piezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the device substrate. One or both mating surfaces may beactivated using, for example, a plasma process. The mating surfaces maythen be pressed together with considerable force to establish molecularbonds between the piezoelectric plate and the device substrate orintermediate material layers.

At 3020, the sacrificial substrate may be removed. For example, thepiezoelectric plate and the sacrificial substrate may be a wafer ofpiezoelectric material that has been ion implanted to create defects inthe crystal structure along a plane that defines a boundary between whatwill become the piezoelectric plate and the sacrificial substrate. At3020, the wafer may be split along the defect plane, for example bythermal shock, detaching the sacrificial substrate and leaving thepiezoelectric plate bonded to the device substrate. The exposed surfaceof the piezoelectric plate may be polished or processed in some mannerafter the sacrificial substrate is detached.

A first conductor pattern, including IDTs and reflector elements of eachXBAR, is formed at 3045 by depositing and patterning one or moreconductor layers on the front side of the piezoelectric plate. All orportions of the first conductor pattern may be over the coating layerformed at 3025. The conductor layer may be, for example, aluminum, analuminum alloy, copper, a copper alloy, or some other conductive metal.Optionally, one or more layers of other materials may be disposed below(i.e. between the conductor layer and the piezoelectric plate) and/or ontop of the conductor layer. For example, a thin film of titanium,chrome, or other metal may be used to improve the adhesion between theconductor layer and the piezoelectric plate. A second conductor patternof gold, aluminum, copper or other higher conductivity metal may beformed over portions of the first conductor pattern (for example the IDTbus bars and interconnections between the IDTs).

Each conductor pattern may be formed at 3045 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, or other etching techniques.

Alternatively, each conductor pattern may be formed at 3045 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 3050, one or more frequency setting dielectric layer(s) may be formedby depositing one or more layers of dielectric material on the frontside of the piezoelectric plate. For example, a dielectric layer may beformed over the shunt resonators to lower the frequencies of the shuntresonators relative to the frequencies of the series resonators. The oneor more dielectric layers may be deposited using a conventionaldeposition technique such as physical vapor deposition, atomic layerdeposition, chemical vapor deposition, or some other method. One or morelithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate. For example, a mask may be used to limit adielectric layer to cover only the shunt resonators.

At 3055, a passivation/tuning dielectric layer is deposited over thepiezoelectric plate and conductor patterns. The passivation/tuningdielectric layer may cover the entire surface of the filter except forpads for electrical connections to circuitry external to the filter. Insome instantiations of the process 3000, the passivation/tuningdielectric layer may be formed after the cavities in the devicesubstrate are etched at either 3010B or 3010C.

In a second variation of the process 3000, one or more cavities areformed in the back side of the device substrate at 3010B. A separatecavity may be formed for each resonator in a filter device. Also, thecavities can be shaped and formed such that two or more resonators canbe on one diaphragm over one cavity. These resonators sharing adiaphragm are acoustically coupled on an acoustic track. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back side of the device substrateto the piezoelectric plate. In this case, the resulting resonatordevices will have a cross-section as shown in FIG. 1.

In a third variation of the process 3000, one or more cavities in theform of recesses in the device substrate may be formed at 3010C byetching the substrate using an etchant introduced through openings inthe piezoelectric plate. A separate cavity may be formed for eachresonator in a filter device. Also, the cavities can be shaped andformed such that two or more resonators can be on one diaphragm over onecavity. The one or more cavities formed at 3010C will not penetratethrough the device substrate. For all cavity forming steps 3010A, 3010B,and 3010C, the dimensions of the cavity will be formed with respect tothe dimensions of the IDTs of the conductor pattern, as described abovewith respect to FIGS. 1, 2A and 2B.

Ideally, after the cavities are formed at 3010B or 3010C, most or all ofthe filter devices on a wafer will meet a set of performancerequirements. However, normal process tolerances will result invariations in parameters such as the thicknesses of dielectric layerformed at 3050 and 3055, variations in the thickness and line widths ofconductors and IDT fingers formed at 3045, and variations in thethickness of the piezoelectric plate. These variations contribute todeviations of the filter device performance from the set of performancerequirements.

To improve the yield of filter devices meeting the performancerequirements, frequency tuning may be performed by selectively adjustingthe thickness of the passivation/tuning layer deposited over theresonators at 3055. The frequency of a filter device passband can belowered by adding material to the passivation/tuning layer, and thefrequency of the filter device passband can be increased by removingmaterial to the passivation/tuning layer. Typically, the process 3000 isbiased to produce filter devices with passbands that are initially lowerthan a required frequency range but can be tuned to the desiredfrequency range by removing material from the surface of thepassivation/tuning layer.

At 3060, a probe card or other means may be used to make electricalconnections with the filter to allow radio frequency (RF) tests andmeasurements of filter characteristics such as input-output transferfunction. Typically, RF measurements are made on all, or a largeportion, of the filter devices fabricated simultaneously on a commonpiezoelectric plate and substrate.

At 3065, global frequency tuning may be performed by removing materialfrom the surface of the passivation/tuning layer using a selectivematerial removal tool such as, for example, a scanning ion mill aspreviously described. “Global” tuning is performed with a spatialresolution equal to or larger than an individual filter device. Theobjective of global tuning is to move the passband of each filter devicetowards a desired frequency range. The test results from 3060 may beprocessed to generate a global contour map indicating the amount ofmaterial to be removed as a function of two-dimensional position on thewafer. The material is then removed in accordance with the contour mapusing the selective material removal tool.

At 3070, local frequency tuning may be performed in addition to, orinstead of, the global frequency tuning performed at 3065. “Local”frequency tuning is performed with a spatial resolution smaller than anindividual filter device. The test results from 3060 may be processed togenerate a map indicating the amount of material to be removed at eachfilter device. Local frequency tuning may require the use of a mask torestrict the size of the areas from which material is removed. Forexample, a first mask may be used to restrict tuning to only shuntresonators, and a second mask may be subsequently used to restricttuning to only series resonators (or vice versa). This would allowindependent tuning of the lower band edge (by tuning shunt resonators)and upper band edge (by tuning series resonators) of the filter devices.

After frequency tuning at 3065 and/or 3070, the filter device iscompleted at 3075. Actions that may occur at 3075 include formingbonding pads or solder bumps or other means for making connectionbetween the device and external circuitry (if such pads were not formedat 3045); excising individual filter devices from a wafer containingmultiple filter devices; other packaging steps; and additional testing.After each filter device is completed, the process ends at 3095.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator, comprising: a piezoelectricplate having front and back surfaces, a portion of the piezoelectricplate forming a diaphragm; and a conductor pattern on the front surface,the conductor pattern comprising a multi-mark interdigital transducer(IDT), fingers of the IDT on the diaphragm, wherein a thickness betweenthe front and back surfaces is greater than or equal to 200 nm and lessthan or equal to 1000 nm.
 2. The acoustic resonator of claim 1, whereinthe piezoelectric plate is lithium niobite or lithium tantalate.
 3. Theacoustic resonator of claim 2, wherein the piezoelectric plate is one ofZ-cut, rotated Z-cut, and rotated Y-cut.
 4. The acoustic resonator ofclaim 1, wherein the IDT is one or more of aluminum, an aluminum alloy,copper, a copper alloy, beryllium, gold, tungsten, and molybdenum. 5.The acoustic resonator of claim 1, wherein a radio frequency signalapplied to the IDT excites a primary acoustic mode within thepiezoelectric plate.
 6. The acoustic resonator of claim 5, wherein theprimary acoustic mode is a shear acoustic mode.
 7. The acousticresonator of claim 1 further comprising the substrate, wherein the backsurface is attached to a surface of the substrate, and wherein thediaphragm spans a cavity in the substrate.
 8. The acoustic resonator ofclaim 7, wherein the substrate is one or more of silicon, sapphire, andquartz.
 9. The acoustic resonator of claim 7, wherein the cavity is arecess in the substrate.
 10. The acoustic resonator of claim 7, whereinthe cavity extends through a thickness of the substrate.
 11. A filterdevice, comprising: a piezoelectric plate having front and backsurfaces, portions of the piezoelectric plate forming one or morediaphragms; and a conductor pattern on the front surface, the conductorpattern comprising a plurality of multi-mark interdigital transducers(IDTs), interleaved fingers of the IDTs on the one or more diaphragms,wherein a thickness between the front and back surfaces is greater thanor equal to 200 nm and less than or equal to 1000 nm.
 12. The filterdevice of claim 11, wherein the piezoelectric plate is lithium niobiteor lithium tantalate.
 13. The filter device of claim 12, wherein thepiezoelectric plate is one of Z-cut, rotated Z-cut, and rotated Y-cut.14. The filter device of claim 11, wherein each of the plurality of IDTsis one or more of aluminum, an aluminum alloy, copper, a copper alloy,beryllium, gold, tungsten, and molybdenum.
 15. The filter device ofclaim 11, wherein a radio frequency signal applied to each of theplurality of IDTs excites a respective primary acoustic mode within thepiezoelectric plate.
 16. The filter device of claim 14, wherein therespective primary acoustic mode is a shear acoustic mode.
 17. Thefilter device of claim 11 further comprising a substrate, wherein theback surface is attached to a surface of the substrate, and wherein theone or more diaphragms spans a respective cavity in the substrate. 18.The filter device of claim 17, wherein the substrate is one or more ofsilicon, sapphire, and quartz.
 19. The filter device of claim 17,wherein the respective cavities are recesses in the substrate.
 20. Thefilter device of claim 17, wherein the respective cavities extendthrough a thickness of the substrate.